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Review

The Role of Progesterone in the Reproductive Physiology of Females of Viviparous Squamata

by
Norma Berenice Cruz-Cano
1,2,
Uriel Ángel Sánchez-Rivera
3,*,
Carmen Álvarez-Rodríguez
3,
Hibraim Adán Pérez-Mendoza
1 and
Martín Martínez-Torres
3,*
1
Laboratorio de Ecología Evolutiva y Conservación de Anfibios y Reptiles, Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Av. de los Barrios 1, Los Reyes Iztacala, Tlanepantla 54090, Estado de México, Mexico
2
Laboratorio de Endocrinología de Peces, Unidad de Morfología y Función, Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Av. de los Barrios 1, Los Reyes Iztacala, Tlanepantla 54090, Estado de México, Mexico
3
Laboratorio de Investigación de Saurios en Asistencia Reproductiva y Desarrollo (LISARD), Unidad de Morfología y Función, Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Av. de los Barrios 1, Los Reyes Iztacala, Tlanepantla 54090, Estado de México, Mexico
*
Authors to whom correspondence should be addressed.
Receptors 2026, 5(1), 8; https://doi.org/10.3390/receptors5010008
Submission received: 30 October 2025 / Revised: 11 February 2026 / Accepted: 25 February 2026 / Published: 27 February 2026
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Versions Notes

Abstract

Progesterone (P4) regulates diverse reproductive processes across vertebrates through nuclear receptors; however, its mechanisms in squamate reptiles—particularly in viviparous species—remain poorly understood. In Squamata, P4 primarily acts through progesterone receptor (PR) isoforms A and B, although relatively few reptilian PR sequences have been characterized to date. Squamate PR exhibits ~50% overall sequence divergence from mammalian homologs yet retains striking conservation in both the ligand and DNA-binding domain across vertebrates. Despite the broadly conserved physiological roles of P4 (folliculogenesis, ovulation, courtship behavior, pregnancy maintenance, and parturition/oviposition), P4 dynamics in viviparous squamates remain unresolved due to heterogeneous circulating hormone concentrations and limited PR phylogeny and structure studies. While mammalian models dominate P4 research due to their biomedical relevance, squamates offer unique evolutionary insights: as the only reptile order exhibiting both oviparity and viviparity within the same clade, squamates represent an ideal model for investigating transitions in parity mode. Elucidating P4 mechanisms in squamates will help bridge this critical evolutionary gap, with important implications for reproductive biology and conservation.

1. Introduction

Comparative endocrinology focuses on the study of endocrine systems, examining their development, regulation, evolution, and the effects of environmental factors on their function across different taxa [1]. The reproductive axis (composed of the hypothalamus, pituitary gland, and gonads) has attracted the attention of many researchers, who have noted a remarkable similarity among vertebrates from the morphological to the molecular level [2,3,4]. This could be of importance for the continuity of the species. However, non-avian reptiles, particularly viviparous species, have received less attention, despite representing an ideal model for comparative studies [5,6] and being a basal group in the evolution of amniotes. The order Squamata has attracted major attention in the study of evolution and reproductive features due to their diversity in the mechanisms of sex determination [7,8], types of reproduction (sexual or asexual) [9], reproductive cycles (seasonal or continuous) [10,11], mating behaviors [12,13,14], and parity modes (oviparous and viviparous) [5,15].
Because of these observations, the concentration of sex steroids such as estradiol (E2), P4, and testosterone (T) has been quantified in many species to elucidate and propose the physiological mechanism that modulates reproduction and even to control reproductive cycles in endangered and commercial species. These hormones are primarily synthesized by the gonads and subsequently released into the bloodstream, where they are transported to a variety of target tissues and bind to their respective receptors. Their production and release occur in response to gonadotropins—namely, follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—which are secreted by the anterior pituitary (hypophysis). The secretion of these gonadotropins, in turn, is tightly regulated by the hypothalamic gonadotropin-releasing hormone (GnRH), establishing a hierarchical endocrine axis that coordinates reproductive function across vertebrate species [16]. However, in Squamata, a distinctive feature in the axis is the presence of a single gonadotropin with dual effects (FSH-LH) that regulates reproductive events [16,17].
Sex hormones have been proposed to act via hormonal pleiotropy (where an element of the endocrine system modifies the phenotype), producing effects like genetic pleiotropy, and influencing the response to selection [18,19] through their effect on genetic covariance [20]. These actions can be classified into organizational and activational effects. The organizational effects take place during early developmental windows—typically before birth or shortly after—and they tend to produce long-lasting or irreversible changes in the structure and function of tissues, particularly within the nervous and reproductive systems. In contrast, the activational effects emerge later in life, most commonly during adulthood, and are typically reversible or transient. These actions depend on the presence and fluctuating levels of circulating hormones and are often associated with the modulation of physiological processes and reproductive behaviors [21].
E2 and P4 are the principal sex steroids involved in the regulation of female reproductive events such as follicular recruitment, follicular development, vitellogenesis, ovulation, and pregnancy. Although numerous efforts have provided a wide conceptual framework to understand the events implicated in the reproduction of diverse species, the seasonality of reproductive cycles and even the extinction risk caused by a modification of these patterns in some reptiles, knowledge of their reproductive endocrinology and physiology is still scarce compared to other vertebrates, such as mammals. Considering the above, our aim is to provide a framework for understanding the influence of P4 on female of Squamata, with particular emphasis on its role in viviparous species throughout the different stages of their reproductive biology. It should be noted that the role of this hormone in eutherian mammalian reproduction has been extensively documented [22,23,24,25], including studies on receptor localization [26] and structure [27,28], as well as in oviparous reptile species, with particular emphasis on turtles [29].

2. P4 and PR

P4 is a steroid hormone containing 21 carbon atoms, synthesized from cholesterol transported into the mitochondria, where the enzyme cytochrome P450 side-chain cleavage (P450scc; CYP11A1) catalyzes its conversion into pregnenolone. This precursor is subsequently metabolized in the smooth endoplasmic reticulum by the enzyme Δ5−4 3β-hydroxysteroid dehydrogenase, resulting in the production of this hormone [1]. This biosynthetic process represents a crucial step in steroidogenesis, as P4 serves not only as a central regulator of reproductive physiology but also as a metabolic precursor for the synthesis of other steroid hormones, including glucocorticoids, mineralocorticoids, and androgens. It has widespread effects, regulating a variety of reproductive functions [21] in different tissues, such as ovary, adrenals, yolk and placenta [30,31,32,33,34,35,36]. There is evidence that steroid hormone signaling systems can undergo evolutionary changes in a tissue-specific manner, and in some cases these modifications appear to facilitate behavioral diversification and adaptation by altering receptor expression, ligand sensitivity, or downstream signaling pathways in different target tissues [21]. In females, it is well-known that P4 influences follicular development, mating behavior [37,38,39], and ovulation [29,40], and, in viviparous species, is indispensable for pregnancy recognition, maintenance and parturition [36,41,42,43].
The multiple effects of P4 are mediated through genomic and non-genomic signaling [44]; however, the latter has not been widely studied in Squamata. In genomic signal transduction, this progestogen exerts its effects by activating nuclear receptors, inducing dimerization to allow for binding to promoter regions and activating gene expression [45,46]. There are five isoforms of the receptor (A, B, C, S, and M), all of which are encoded by a single gene [47]. Although isoforms C, S, and M have been reported in other vertebrates, their presence and functional relevance in squamates remain unconfirmed by the current literature. Isoforms A and B have been the most extensively studied and characterized, particularly in relation to their distinct roles in modulating transcriptional activity and cellular responses [48]. For this reason, the present review primarily focusses on the functional relevance and regulatory mechanisms associated with receptor isoforms A (PRA) and B (PRB). Both PRA and PRB are transcribed from the same gene and consist of a N-terminal region that stimulates gene transcription (domain A/B), a DNA-binding domain (C), a nuclear localization signal (D), and a ligand-binding region (E) [23] (Figure 1). The difference between the two receptors lies in an additional region with activating function (AF) in PRB [45]. Although some sequences of PR are available on the gene bank, there are no studies related to the structure of these receptors, even when the modeling is available on multiple platforms (Figure 1).
Analysis of the available genomic data in Uniprot (https://www.uniprot.org/, accessed on 26 October 2025) reveals that the PR gene in Squamata exhibits a marked degree of sequence divergence compared to other vertebrates, with overall sequence similarity to mammalian PRs averaging ~50% (Figure 2A). However, most of the sequences are predicted for whole-genome sequencing and do not consider the isoform or provide detailed information on the major domains. Within Squamata, interspecific conservation is variable, ranging from ~50% to ~90%, indicating that while some regions of the receptor evolve rapidly, others remain under strong evolutionary constraints. This is particularly evident in the ligand-binding domain and the DNA-binding domain, which show a high degree of conservation across vertebrates (Figure 2B). When aligned, both domains of viviparous squamates (e.g., N. scutatus) retain key residues involved in ligand recognition and activation that are also present in eutherian mammals, suggesting that progesterone-binding and activation mechanisms are conserved despite the broader divergence in other domains. However, variation outside the binding domains may contribute to species-specific regulatory effects. These differences could influence co-regulator recruitment, receptor stability, and transcriptional activation profiles, potentially leading to divergent biological effects on reproduction, implantation, and pregnancy maintenance between squamates and eutherian mammals.
Consistent with this structural conservation, functional studies in reptiles have shown that the P4 antagonist RU486 (mifepristone)—a compound originally developed for mammalian progesterone receptors—elicits clear biological effects. It can affect early pregnancy processes by diminishing intercellular attachment in the lizard Pseudemoia entrecasteauxii [49], whereas no significant effects on uterine epithelium morphology were observed in the lizard Niveoscincus conventryi [50]. Although direct binding affinity measurements for RU486 or synthetic progestins in the PR of squamates are currently lacking, these physiological responses provide indirect evidence of receptor cross-reactivity, likely mediated by conserved ligand-binding domains. To the best of our knowledge, studies related to phylogeny, and particularly to protein structure and functionalization in this group, are lacking. Future research into the structural characteristics and differences in less constrained domains of this group could contribute to understanding the diverse reproductive adaptations and strategies unique to viviparous of Squamata.
Although advances in sequencing technologies have facilitated the identification of receptor isoforms in several Squamata species (Table 1), the functional consequences of these variants and their effects remain poorly explored and the number of viviparous species is minor compared to the oviparous. Determining whether structural divergence within specific receptor domains correlates with differential localization or activation patterns could provide key insights into how hormonal signaling pathways have diversified in response to reproductive demands. Thus, integrating structural biology with cell-specific expression analyses becomes essential to elucidate how these receptors operate within the complex physiological context of viviparity.
These receptors show both cytoplasmic and nuclear localization in the cells of many tissues. A membrane localized progestin receptor has also been reported [47], demonstrating high functional plasticity due to the large number of possible combinations of modulators, activity levels, and responses [60]. Also, other factors, such as cAMP, gonadotropins and cyclins, can regulate the expression of PR [60]. The multiple isoforms reported and their effects are similar across taxa; however, their quantity and localization are species-specific, and their presence is linked to diverse patterns of circulating sex steroids. In eutherian mammals, for example, PRA increases with P4, while PRB responds to E2 in the uterus [45], endometrium, and in the pituitary and brain, breast and other tissues [60].
Although the number of sequences reported of PR in reptiles is scarce, both isoforms have been reported (Table 2) and are associated with different stages of the reproductive cycle [29,49,56,59]. It should be noted that these studies use antibodies raised for other species that were shown to be useful in detecting the changes associated with the reproductive cycle. Studies with more specific markers may help to discern the effects of each isoform.
The pattern of P4 secretion in most viviparous of Squamata shows an increase at the end of follicular development, a decrease after ovulation, and a subsequent rise due to the formation of the corpus luteum (CL) [29,43,64,65]. These concentrations are then maintained during pregnancy, when the placenta acquires steroidogenic activity [41,42,43,66]. However, a generalized model cannot be provided due to the heterogeneity of the reported data (Table 3).

3. Progesterone and Follicular Development

The role of this sex steroid in the early stages of follicular development remains to be elucidated. In the early stages, low concentrations of P4 enhance follicular recruitment in species such as Boa constrictor [92] and Mabuya mabuya [62], among others [16,29,93]. However, it remains unclear whether their effects are exerted directly (as a progestogen) or indirectly (converted into an estrogen), and if, for instance, the steroid exerts its role through a follicular cells-derived factor [94]. The mechanism that has been proposed for P4 regulation in this stage suggests a binding to ‘somatic’ receptors that induce the secretion of a diffusible paracrine factor that targets the activity of germinal beds, oogonial proliferation, and oocytes recruitment for follicular development in the lizard P. siculus [56].
During the vitellogenesis, its autocrine and paracrine effects on the ovarian follicles modify the number of PRs [29,76]; in this way, it can influence the yolk uptake and synthesis. This can be achieved through the regulation of gonadotropin secretion via gonadotropin-releasing hormone activity [56]. However, our understanding of reptile reproductive neuroendocrinology is insufficient to explain the endocrine and paracrine factors modulating yolk uptake. Also, study of the steroidogenic activity and the circulating hormones may help to establish if the role of P4 in vitellogenesis is more related to its conversion to E2. In the lizard Uromastix acanthinura, PRA and PRB are positively influenced by E2, which highlights the heterogeneity in responses between species [59]. In the turtle Chrysemys picta, PRA expression inhibits vitellogenin (VTG) synthesis [95] while PRB promotes it [96]. It remains to be elucidated if a similar regulation is present in viviparous of Squamata.
As the cycle continues, a subtle increase in P4 promotes ovulation [90]. A similar effect is observed in mammals, where an imprinting with E2 promotes the LH surge and the follicle’s response to this signal [97]. After ovulation, the primary source of P4 is the CL [41,42,43]. Additionally, a regulation of clutch size via follicular atresia, due to the negative feedback of P4 on gonadotropin secretion, has been proposed [98,99].

4. Progesterone and Pregnancy

It is proposed that P4, as in other viviparous vertebrates, plays an important role in pregnancy maintenance. The steroidogenic activity of granulosa–luteal cells from the CL maintains P4 concentrations during the early stages of pregnancy [43,82]. It should be noted that the lifespan of the CL during pregnancy depends on several factors, including the sources of extraovarian P4 (such as atretic follicles, adrenals and placenta) [62,65,66], the length of the pregnancy [100], and, in some cases, the presence of an embryo [42,101]. Although luteolysis occurs at different stages of pregnancy [42,43] in many viviparous lizards, such as Chalcides chalcides [65], S. jarrovi [88], S. mucronatus [89], B. imbricata imbricata [42,43], and lizards of the genus Mabuya, the concentration of P4 does not decrease afterward, and in some cases, it even increases. However, lutectomy experiments have demonstrated that the CL influences the P4 concentration, gestation [49] and parturition [42,89].
Although a generalized pattern of secretion cannot be established, in diverse species, progestogen concentrations are high in the early stages of pregnancy and decrease before parturition (Table 3). However, more studies considering the source of this hormone and its possible role in maternal recognition of pregnancy could shed light on the mechanisms involved in the transition to viviparity. The dynamics of this hormone are similar to those observed in oviparous species, as viviparity is considered to have evolved from an ancestral oviparous state [15,102,103].
Progesterone secretion is essential for viviparity due to its multiple roles over space and time, ensuring embryo development through its effects on uterine epithelium proliferation. This is fundamental to promote the interaction between the mother and the embryos in development via diverse mechanisms such as hypervascularization to increase gas exchange, the inhibition of uterine contractility to promote egg retention, and the limitation of follicular development by reducing gonadotropin secretion and exerting antiestrogenic activity [29,43,76,104,105].
As pregnancy progresses, the demand for maternal-derived compounds changes with the development of the placenta, which is reflected in the modification of P4 synthesis and secretion [76,88,102,106]. Primitive placentas exhibit steroidogenic activity during the early stages of luteolysis and continue this activity until the end of gestation [65,88,107,108]. However, other sources, such as ovarian follicles and the adrenals, may also contribute to steroidogenesis [62]. Changes in sex steroid concentrations modify the characteristics of the uterine epithelium—such as its height, surface structure, number of ciliated cells, and arrangement of microvilli—along with its secretory activity and contractility [5,17,29,35,109].
During embryonic development, the induction of immunomodulatory mechanisms is necessary to prevent embryo rejection. This is achieved by an increase in P4 concentrations, which diminish the activity of complement proteins and facilitate fetal–maternal interaction [68]. Another effect of this progestogen is the maintenance of uterine epithelium morphology, reducing its susceptibility to bacterial infections by modifying cellular tight junctions [29,69,110]. In the later stages of pregnancy, a decrease in P4 concentrations increases the presence of arginine–vasotocin receptors, promoting labor [29,42,69,110]. The role of this hormone in parturition remains to be elucidated; an active role of the embryo is suggested since the decrease in P4 does not influence the timing of birth [76]. While P4 predominantly supports pregnancy maintenance through its physiological actions on the uterus and placenta, declining concentrations later in gestation also trigger behavioral transitions essential for successful reproduction. As luteolysis progresses and P4 levels decrease, this hormonal shift also modulates female receptivity and courtship behaviors— limiting sexual receptivity post-partum to initiate follicular development and regulating the maternal care patterns observed in certain squamate species. These dynamic P4-mediated transitions underscore its multifaceted role across both physiological and behavioral reproductive contexts.

5. P4 and Courtship Behavior

Mating behavior is regulated by sex steroids, and while the effects of estrogens and androgens have received more attention, P4 participation has been less studied. However, it has been demonstrated that it also influences behaviors such as mating, copulation, territoriality, and nesting [111]. Sexual dimorphism is widely expressed across many species, with males generally exhibiting more striking coloration, which is associated with testosterone and P4 concentrations [40,112,113,114,115,116]. A similar pattern is observed in female lizards of S. virgatus, S. phyrocephalus, and Ctenophorus maculatus [112,113,114,115,116], which are signals of female receptivity in L. vivipara [110]. In snakes of the genus Crotalus, a decrease in P4 maintains contraction of the posterior uterus, facilitating sperm storage to ensure fertilization, as reproductive cycles are often dissociated [71].
Although few reptiles exhibit parental care, in Egernia lizards [117] and Crotalinae snakes [118,119], an increase in P4 induces maternal permanence with young until the first molt. However, the mechanisms regulating this behavior remain unknown.

6. Conservation Implications

Comprehensive studies on reproductive biology are essential for identifying the constraints that hinder the persistence of herpetofauna, especially given the remarkable diversity in reproductive modes, which remain poorly studied in viviparous species. Characterizing hormonal profiles and elucidating their regulatory mechanisms enables both the assessment of how environmental stressors affect reproductive success and the controlled induction of follicular development and ovulation. This knowledge not only facilitates the direct management of threatened species but also informs predictions of their resilience to climate change and guides the prioritization of conservation efforts based on ecophysiological vulnerability. Furthermore, these advances are instrumental for monitoring wild populations and applying assisted reproductive technologies, as well as for designing more efficient captive breeding programs.

7. Conclusions

Currently, the number of taxa studied from an endocrinological perspective is increasing, leading to broader comparative studies. This is partly due to the degree of similarities in the components that regulate reproductive events. While the role of P4 has been extensively studied in mammals, Squamata models offer valuable insights for identifying the origins of various pathways and events involved in reproduction. These species exhibit unique characteristics, such as polymorphism in follicular cells, the proliferation of germinal beds, and varying degrees of placentation, which could help elucidate the role of P4 and the embryo in the evolution of viviparity. Understanding the mechanisms of action of sex steroids and their receptors, the involvement of environmental signals, and the mechanisms of desensitization could be useful in conservation biology, enhancing management programs through induced reproduction, particularly in endangered species.

Author Contributions

Conceptualization, N.B.C.-C. and M.M.-T.; investigation, N.B.C.-C.; formal analysis, N.B.C.-C., C.Á.-R., U.Á.S.-R. and H.A.P.-M.; visualization, N.B.C.-C. and U.Á.S.-R.; writing and original draft preparation, N.B.C.-C. and U.Á.S.-R.; writing, review and editing, N.B.C.-C., C.Á.-R., U.Á.S.-R., M.M.-T. and H.A.P.-M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Programa de Investigadoras e Investigadores of the Consejo Mexiquense de Ciencia y Tecnología (COMECYT), through grant CAT2024-0034 awarded to N.B.C.-C.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

N.B.C.-C. acknowledges the Consejo Mexiquense de Ciencia y Tecnología (COMECYT) for supporting this research CAT2024-0034. We express our gratitude to the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AFActivating function
CLCorpus luteum
E2Estradiol
FSHFollicle-stimulating hormone
LBDLigand-binding domain
LHLuteinizing hormone
P4Progesterone
PRA, PRBProgesterone Receptor A, B
VTGVitellogenin

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Receptors 05 00008 g001
Figure 1. Progesterone exerts its genomic effects through binding to nuclear receptors. (A) Predicted PR structure of Notechis scutatus (UniProt: A0A6J1US99). (B) Predicted PRA and PRB isoforms reported in Squamata. PRB is associated with the stimulation of vitellogenesis, whereas PRA is associated with its inhibition during pregnancy. (C) Progesterone dynamics across Squamata reproductive cycles are highly variable, precluding a single generalized model for the order. CTE: carboxyl–terminal extension.
Figure 1. Progesterone exerts its genomic effects through binding to nuclear receptors. (A) Predicted PR structure of Notechis scutatus (UniProt: A0A6J1US99). (B) Predicted PRA and PRB isoforms reported in Squamata. PRB is associated with the stimulation of vitellogenesis, whereas PRA is associated with its inhibition during pregnancy. (C) Progesterone dynamics across Squamata reproductive cycles are highly variable, precluding a single generalized model for the order. CTE: carboxyl–terminal extension.
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Figure 2. (A) Percent identity matrix of the PR gene across diverse taxa. The full-length sequence was used in this analysis. (B) Alignment and conservation of the DNA-binding (567–640) and ligand-binding domain (688–933) regions of the progesterone receptor (PR) in various vertebrates. Binding domains from fish (Danio rerio, UniProt ID: C9VN37), amphibians (Rana debowskii, UniProt ID: Q8AYI2), reptiles—Squamata (N. scutatus [viviparous], UniProt ID: A0A6J1US99; Pantherophis guttatus, UniProt ID: A0A6P9B9Z2; Salvator merianae, UniProt ID: A0A8D0C3X2), birds (Gallus gallus, UniProt ID: P07812), and mammals (Homo sapiens, UniProt ID: P06401) were aligned using Geneious Prime (Version 2025.2). An asterisk indicates a viviparous species of Squamata.
Figure 2. (A) Percent identity matrix of the PR gene across diverse taxa. The full-length sequence was used in this analysis. (B) Alignment and conservation of the DNA-binding (567–640) and ligand-binding domain (688–933) regions of the progesterone receptor (PR) in various vertebrates. Binding domains from fish (Danio rerio, UniProt ID: C9VN37), amphibians (Rana debowskii, UniProt ID: Q8AYI2), reptiles—Squamata (N. scutatus [viviparous], UniProt ID: A0A6J1US99; Pantherophis guttatus, UniProt ID: A0A6P9B9Z2; Salvator merianae, UniProt ID: A0A8D0C3X2), birds (Gallus gallus, UniProt ID: P07812), and mammals (Homo sapiens, UniProt ID: P06401) were aligned using Geneious Prime (Version 2025.2). An asterisk indicates a viviparous species of Squamata.
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Table 1. Species of Squamata with available progesterone receptor sequences.
Table 1. Species of Squamata with available progesterone receptor sequences.
Species ParityLengthUniprot Entry
Anolis carolinensis (Green anole)oviparous402[51]
Aspidoscelis inornatus
(Little striped whiptail)		oviparous902[52,53,54]
Aspidoscelis uniparens
(Desert grassland whiptail lizard)		oviparous348[55]
Crotalus adamanteus
(Eastern diamondback rattlesnake)		viviparous1074[56,57]
Eublepharis macularius
(Leopard gecko)		oviparous793[49]
Uromastyx acanthinuraoviparous384[58,59]
Naja naja
(Indian cobra)		oviparous849A0A8C6XTF0_NAJNA
N. scutatus
(Mainland tiger snake)		viviparous869A0A6J1US99_9SAUR
P. guttatus
(Corn snake)		oviparous886A0A6P9B9Z2_PANGU
Phrynocephalus forsythiiviviparous848A0A9Q0XMF7_9SAUR
Podarcis lilfordi
(Lilford’s wall lizard)		oviparous894A0AA35P2N6_9SAUR
Podarcis muralis
(Wall lizard)		oviparous831A0A670IQT2_PODMU
Pogona vitticeps
(Central bearded dragon)		oviparous838A0ABM5FWE3_9SAUR
Pseudonaja textilis
(Eastern brown snake)		oviparous882A0A670Y4G1_PSETE
S. merianae
(Argentine black and white tegu)		oviparous889
869		A0A8D0C3X2_SALMN
A0A8D0BVF9_SALMN
Varanus komodoensis
(Komodo dragon)		oviparous885
853		A0A8D2JIX4_VARKO
A0A8D2JFL5_VARKO
Table 2. Tissue-specific localization of progesterone receptors in viviparous squamate species.
Table 2. Tissue-specific localization of progesterone receptors in viviparous squamate species.
Species Progesterone Receptor LocationReferences
A. carolinensisPreoptic area[51]
Cnemidophorus inornatus and C. uniparensCell nucleus of the preoptic area of the infundibular and ventromedial hypothalamus[52,53,54]
Hemidactylus flaviviridisThe epithelial and glandular layer of the oviduct
Cytoplasm and nuclei of theca interna and granulosa cells in vitellogenic and previtellogenic follicles.
Granulosa cells of the corpus luteum		[61]
Mabuya sp.Nucleus and cytoplasm of theca and granulosa cells of previtellogenic follicles, corpus luteum, columnar cells, lamina propria connective tissue, uterine epithelial cell cytoplasm and myometrium, muscle fibers, connective tissue, and uterine epithelium of placenta[62]
Nerodia sp.Cytosol and nucleus of oviduct cells[55]
Podarcis siculusLiver in gonadal quiescence stage and oviduct[56,57]
P. entrecasteauxiiCell nucleus in uterine epithelium[49]
Sceloporus torquatusNucleus and cytoplasm of pyriform cells, the oocyte cortex small cells and yolk[63]
U. acanthinuraRPA in the cytosol of luteal phase oviduct cells
RPB in the nucleus of hepatocytes		[58,59]
Table 3. Plasma progesterone concentrations (ng/mL) across the reproductive cycle of viviparous squamata species. Modified from Albergotti and Guillette (2011) [67].
Table 3. Plasma progesterone concentrations (ng/mL) across the reproductive cycle of viviparous squamata species. Modified from Albergotti and Guillette (2011) [67].
SpeciesFollicular Development *Pregnancy *References
Snakes
Agkistrodon piscivorus E: 8.55 ± 0.66
M: 1.11[35,68]
L: 3.8 ± 0.5
PP: 0.7 ± 0.05
Crotalus atroxPV: 0.29E: 17.85 ±1.14[69,70]
V: 1.17 ± 0.43M: 9.47 ± 0.66
PO: 3.63 ± 0.73L: 0.74
O: 27.62 ±2.28PP: ND
C. durissus terrificusV: 2.55 ± 1.17M: 25.34 ± 2.77[71]
PO: 4.02 ± 1.45PP: 6.16 ± 2.56
C. oreganusPV: 0.6 ± 0.1 [72]
V: 1.2 ± 0.1
PO: 1.2 ± 0.1
O: 0.6 ± 0.04
Nerodia sipedonV: 1.27 ± 0.19E: 4.95 ±1.41[73]
PO: 3.93 ± 0.83M: 6.94 ± 0.78
L: 2.81 ± 0.44
Nerodia taxispilotaV: 0.44 ± 0.04E: 1.93 ± 0.24[73]
PO: 0.91 ± 0.08
Thamnophis elegans E: 1.70 ± 0.30[66]
M: 6.20 ±1.0
L: 1.0 ± 0.2
T. sirtalis parietalisNDND[74]
Trimeresurus stejnegeriPV, V, PO: ND
O: 20 ± 12		ND[75]
Vipera aspisPV: 4 ± 1M: 19.2 ± 2.3[76]
V: 6.8 ± 0.8L: 10.3 ± 0.5
PO: 15.3 ± 5.1PP: 5.9 ± 0.43
O: 12.1 ± 0.5
Lizards
Barisia imbricata E: 3.07 ± 1.04[43]
M: 1.31 ± 0.32
L: 0.74 ± 0.24
Bradypodion pumilumV: 0.86E: 4.95 ± 3.9[77]
PO: 0.95 ± 0.71M: 2.30 ± 0.34
Lacerta viviparaPO: 57.90 ± 4.8M: 216.22 ± 17.92[78]
L: 347.20 ± 17.38
PP: 3.71 ± 0.56
Mabuya sp.V: 1.6 ± 0.13E: 14.1 ± 0.63[79]
L: 4.4 ± 0.41
PP: 3.5 ± 0.25
Niveoscincus metallicusPV: NDE: 9.1 ±1.33[80,81]
V: 1.8M: 11.5 ± 1.93
L: 4.3 ± 1.7
N. microlepidotusPO: 38.5 ± 7.9E: 15.4 ± 5.9[32,82,83,84]
L: 1.1 ± 0.2
PP: 2.7 ± 0.9
N. ocellatusPV: 0.5 ± 0.1E: 5.1 ± 0.7[84]
V: 1.8 ± 0.3M: 5.3 ± 1.8
PO: 2.1 ± 0.3L: 6.5 ± 1.5
PP: 1.6 ± 0.22
Oligosoma maccaniPV: 1 ± 0.1E: 3.5 ± 0.4[85]
V: 1 ± 0.1M: 15.3 ± 2.5
L: 6.30 ± 2
Sceloporus cyanogenysV: 0.70 ± 0.15E: 3.302 ± 0.48[86]
PO: 0.90 ± 0.38L: 3.5 ± 0.34
PP: 1.6 ± 0.22
S. jarrovi E: 0.75 ± 0.11[87,88]
M: 0.95 ± 0.05
L: 4.1 ± 0.94
PP: 0.43 ± 0.06
S. mucronatus E: 0.48 ± 0.15[89]
M: 2.65 ± 0.3
L: 0.36 ± 0.09
S. torquatusPV: 1.79 ± 0.36E: 2.06 ± 0.28[63]
V: 1.59 ± 0.21M: 1.68 ± 0.24
PO: 1.46 ± 0.18L: 1.69 ± 0.47
Tiliqua nigroluteaPV: 1.10 ± 0.2E: 8.85 ± 1.17[90]
V: 1.28 ± 0.2M: 12.8 ± 1.27
PO: 1.92 ± 0.2L: 4.48 ± 0.96
O: 6.40 ± 0.1
PsO: 5.33 ± 0.6
T. rugosa E: 0.57 ± 0.07[64,91]
M: 2.07 ± 0.39
L: 0.34 ± 0.10
PP: 0.14 ± 0.01
* Values represent the mean and standard error of the mean. PV: previtellogenesis; V: vitellogenesis; PO: preovulatory; O: ovulatory; PsO: post ovulatory; E: early; M: middle; L: late pregnancy; PP: postpartum stages. ND: not detectable.
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Cruz-Cano, N.B.; Sánchez-Rivera, U.Á.; Álvarez-Rodríguez, C.; Pérez-Mendoza, H.A.; Martínez-Torres, M. The Role of Progesterone in the Reproductive Physiology of Females of Viviparous Squamata. Receptors 2026, 5, 8. https://doi.org/10.3390/receptors5010008

AMA Style

Cruz-Cano NB, Sánchez-Rivera UÁ, Álvarez-Rodríguez C, Pérez-Mendoza HA, Martínez-Torres M. The Role of Progesterone in the Reproductive Physiology of Females of Viviparous Squamata. Receptors. 2026; 5(1):8. https://doi.org/10.3390/receptors5010008

Chicago/Turabian Style

Cruz-Cano, Norma Berenice, Uriel Ángel Sánchez-Rivera, Carmen Álvarez-Rodríguez, Hibraim Adán Pérez-Mendoza, and Martín Martínez-Torres. 2026. "The Role of Progesterone in the Reproductive Physiology of Females of Viviparous Squamata" Receptors 5, no. 1: 8. https://doi.org/10.3390/receptors5010008

APA Style

Cruz-Cano, N. B., Sánchez-Rivera, U. Á., Álvarez-Rodríguez, C., Pérez-Mendoza, H. A., & Martínez-Torres, M. (2026). The Role of Progesterone in the Reproductive Physiology of Females of Viviparous Squamata. Receptors, 5(1), 8. https://doi.org/10.3390/receptors5010008

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