Twenty-One Mayfly Gynandromorphic Cases from China

Abstract

Gynandromorphism of mayfly has unique biological and scientific value. However, most cases of previous studies on this phenomenon were from Europe and America, with only two cases reported from China. In this study, we examined the mayfly collections in our laboratory and obtained 21 intersex specimens belonging to three families (Baetidae, Leptophlebiidae, and Siphlonuridae) and seven species (Baetis rutilocylindratus, three unnamed Baetis species, Neoleptophlebia sp., Choroterpes facialis, and Siphlonurus lacustris), which greatly enrich the diversity of mayfly gynanders of China. Biogeographically, this phenomenon appears cosmopolitan. Regarding the potential causes of mayfly gynandromorphism, we hypothesize that inorganic factors may play a significant role.

1. Introduction

Gynandromorphism is a relatively unique phenomenon in nature and has been reported in 16 insect orders previously, including Ephemeroptera [1,2,3,4]. The first case in mayfly was found by Lestage [5], while Soldán and Landa [1] provided systematic research on this issue, including 26 species from seven families in Europe and South America at the time. However, Asian cases remain rare and unevenly documented, with only five reports published to date [1,4,6,7,8].

To date, approximately 70 wild mayfly specimens exhibiting both male and female characteristics have been recorded worldwide. Most reports involve the Baetidae, Heptageniidae, and Leptophlebiidae families, with fewer reports from Ameletidae, Ephemerellidae, Ephemeridae, Polymitarcyidae, and Potamanthidae [1,3,6,7,9,10,11,12,13]. In contrast, all Asian records are from the three most diverse families, with no other families involved [1,4,6,7,8].

Based on the degree of development and distribution of male and female sexual characteristics, Soldán and Landa [1] classified the reported mayfly individuals into three types: true gynandromorphism (male and female parts of body normally developed, only exceptionally reduced in size, evenly or unevenly distributed), intersexuality (male and/or female parts of body intermediately developed, always unevenly distributed) and external genitalia teratology (deformations or unusual location of forceps and penis lobes). Yang et al. [4] reported the first dominant male in the intersexuality type, suggesting that more unusual types of gynandromorphism may exist.

The five Asian gynandromorphisms were found in Mongolia, Tajikistan, India, and China, including only two Oriental realm cases [4,8]. This geographic bias is believed to result from a shortage of research.

In China, only two records have been reported, showing great differences [4,6]. They come from different regions and families. Greater diversity will likely be revealed as more cases are documented.

In recent years, stimulated by the previous research of Yang et al. [4], we systematically checked our collection for more gynandromorphisms. From our investigations, we found 21 gynandromorphic individuals, including seven species in three families. These findings not only increase the number of documented cases of Chinese and Asian gynandromorphism but also offer further insights into the types and nature of this phenomenon.

2. Materials and Methods

All specimens in this study were found in our accumulated collections, attracted and gathered by light trap, and preserved in 75–90% ethanol.

Identifications were based on morphology, following Zhou et al. [14].

Digital photos were taken using a digital camera (Mshot MSX11, single-lens reflex, Mingmei Optoelectronics, Guangzhou, China) or a digital camera (Mshot MZ81, Mingmei Optoelectronics, Guangzhou, China) connected to a stereo microscope. The obtained images were edited and processed by Adobe Photoshop 2022.

The specimens are in the Mayfly Collection, College of Life Science, Nanjing Normal University (NNU), China.

3. Results

3.1. Description

In total, 21 gynandromorphic specimens were found using the criteria proposed by Soldán and Landa [1]. They represent seven species in three families (

Table 1. Information and main features of 21 gynandromorphic mayflies from China.

Individual	Stage	Name	Family	Collected Province	Sex (Judging from the Content in Abdomen)	Main Feature
1	imago	Baetis rutilocylindratus	Baetidae	Zhejiang	female	Predominantly female with male compound eyes
2	subimago	Baetis sp.	Baetidae	Hunan	male	Predominantly male with right female eye, left foreleg shorter than right
3	subimago	Baetis sp.	Baetidae	Hubei	female	Predominantly female with right male eye
4	subimago	Baetis sp.	Baetidae	Guizhou	female	Predominantly female with left male eye and a relic forceps
5	subimago	Neoleptophlebia sp.	Leptophlebiidae	Anhui	female	Predominantly female with right male eye
6	imago	Choroterpes facialis	Leptophlebiidae	Fujian	male	Predominantly male with left female eye, without forceps but with slightly degenerated penes
7	imago	Choroterpes facialis	Leptophlebiidae	Fujian	female	Predominantly female with right male eye and degenerated penes
8–21	imago	Siphlonurus lacustris	Siphlonuridae	Inner Mongolia and Heilongjiang	male	Predominantly male with female eyes, forceps degenerated in 13 individuals and missing in one

). The habitus of the gynandromorphic mayflies are shown in Figure 1.

(1) Individual 1: Baetis rutilocylindratus (Wang et al., 2011) [15]

Collecting information: gynandromorph imago, Tangzhong Village, Pingyang County, Zhejiang Province, 27.566855° N, 120.502847° E, VIII-12-2023, leg. De-Wen Gong.

Description: predominate female imago, body symmetrical, uniformly reddish in coloration, but compounded eyes clearly divided into upper orange turbinate part and oval dark lower part, exhibiting a masculinized morphology (Figure 1A and Figure 2A); forelegs subequal to mid- and hindlegs in length (Figure 1A); abdomen filled with eggs inside (Figure 2B); with typical posterior end of normal female (Figure 2C).

(2) Individual 2: Baetis sp.

Collecting information: gynandromorph subimago, Huayuan Village, Pingyang County, Hunan Province, 28.572030° N, 109.482078° E, VII-11-2009, leg. Peng Li.

Description: predominate male subimago, but left compound eye with pale turbinate upper portion and dark spherical lower portion, bigger than the right one (Figure 1B and Figure 3A); two forelegs dissimilar: the left one shorter than the right one, like female foreleg (Figure 1B and Figure 3A); abdominal sternum IX similar to regular female (Figure 3B).

(3) Individual 3: Baetis sp.

Collecting information: gynandromorph subimago, Qingyan Village, Gaojiayan Town, Changyang County, Hubei Province, 30.599252° N, 111.054555° E, VII-20-25-2013, leg. Yan-Xia Wang.

Description: predominate female subimago, body symmetrical except the compound eyes: right eye divided into deep orange turbinate upper part and black semi-spherical lower part, significantly larger than the left one, which is dark in color and spherical in shape (Figure 1C and Figure 4A); eggs contained in the abdomen (Figure 4B); posterior end without any traces of male genitalia (Figure 4C).

(4) Individual 4: Baetis sp.

Collecting information: gynandromorph subimago, Leishan County, Kaili City, Guizhou Province, 26.063334° N, 108.189377° E, VII-14-2009, leg. Jia-Chang Liang.

Description: predominate female subimago, but left half of the head with a male compound eye, which divided into yellowish-white cylindrical upper part and dark globular lower portion; left eye dark and spherical (Figure 1D and Figure 5A); abdomen: fully filled with eggs (Figure 5B); genitalia: base of the forceps on the left side of abdominal segment IX present (Figure 5C).

(5) Individual 5: Neoleptophlebia sp.

Collecting information: gynandromorph subimago, Huangshan Wild Monkey Valley, Fuxi Village, Anhui Province, 29.707600° N, 118.306231° E, VII-21-2021, leg. Bei-Xin Wang.

Description: predominate female subimago, symmetrical body except large right compound eye with orange turbinate upper portion and dark spherical lower portion, same as the typical eye of male (Figure 1E and Figure 6A); abdomen stuffed with eggs (Figure 6B); abdomen without male genitalia, similar to the normal female (Figure 6C).

(6) Individuals 6 and 7: Choroterpes facialis (Gillies, 1951) [16]

Collecting information: 2 gynandromorph imagoes, 201 county road, Nanping City, Fujian Province, 27.837329° N, 117.969880° E, alt. 283.4 m, VII-20-2022, leg. Xin-He Qiang.

The specimens were reported in detail by Yang et al. [4].

Individual 6: predominant male imago, but with a female-like left compound eye: uniformly black and semi-spherical in overall shape (Figure 1F and Figure 7A); lacking forceps; penes slightly degraded (Figure 7B).

Individual 7: predominant female imago, with eggs inside its body (Figure 1G and Figure 7D); right compound eye is a typical male one, relatively large, divided into orange turbinate dorsal portion and dark semi-spherical ventral portion; left eye female-like (Figure 7C); penes clearly degraded (Figure 7D).

(7) Individuals 8–21: Siphlonurus lacustris (Eaton, 1870) [17]

Collecting information: 6 gynandromorph imagoes, Genhe River, Inner Mongolia, 50.230000° N, 120.220000° E, VIII-10-2007, leg. Shi-Lei Wang; 8 gynandromorph imagoes, Laocao River, Mohe, Heilongjiang Province, 52.483070° N, 122.422002° E, VII-16-19-2024, leg. Xin-He Qiang.

Description: 14 predominant male imagoes but compound eyes purplish-gray globular, smaller than normal male but similar to normal female (Figure 1H–U and Figure 8A–C); forelegs longer than the mid- and hindlegs, but the degree of elongation not as great as that of a normal male (Figure 9A,B); genitalia: penis present; forceps degraded into different size, apparently shorter than normal situation (Figure 8D–G), completely lack in one individual, which closely resembles a typical female (Figure 8H,I).

3.2. Summary Information

Among 21 individuals, four are in the family Baetidae, three individuals belong to the family Leptophlebiidae, and 14 imagoes come from Siphlonuridae. There are dominant characteristics of both males and females (

Table 2. Summary data of mayfly gynandromorphs in this study.

Information	Data
Family	3 (Baetidae, Leptophlebiidae, Siphlonuridae)
Number of species	7 (Baetidae 4, Leptophlebiidae 2, Siphlonuridae 1)
Number of individuals	21 (Baetidae 4, Leptophlebiidae 3, Siphlonuridae 14)
sex	Male and female in Baetidae and Leptophlebiidae, male in Siphlonuridae
stage	Subimago and imago
type	Dominant male (17) or dominant female (4)
Distribution	North to South China

).

4. Discussion

Soldán and Landa [1] identified three key patterns in the occurrence of mayfly intersexes: (1) the number of reported mayfly gynandromorphic specimens in a family is related to the number of species within it; (2) The occurrence probability of gynandromorphism is also associated with the degree of sexual dimorphism of that family; and (3) the occurrence of gynandromorphism in mayflies is not geographically constrained. Our results support the last pattern but contradict the first two.

First, gynandromorphism appears more frequently in Siphlonuridae in our data. In two collections, we identified 14 intersexes, more than in any other. However, the family Siphlonuridae is a relatively small one, including four genera and about 50 species only [7]. Except for this family, other gynandromorphisms found in China fall into the three most diverse families, Baetidae, Leptophlebiidae, and Heptageniidae. In terms of species number, our results support the conclusion of Soldán and Landa [1] well: four species in the Baetidae, two in the Leptophlebiidae, but only one in the Siphlonuridae. Wu et al. [6] reported another gynandromorph of China in the family Heptageniidae.

The second exception is the sexual dimorphism. Just like the Heptageniidae, the males of Siphlonuridae differ from females by larger compound eyes, longer forelegs, and male genitalia in external morphology, but the color of their eyes is always dark, unlike Baetis or Leptophlebia. However, the greater number of Siphlonurus individuals in our study breaks the second conclusion of Soldán and Landa [1], at least in population quantity.

The third point to make is gender. In previous reports, the intersexes were almost females [1,4,7,8,11]. Yang et al. [4] first found the dominant male in the family Leptophlebiidae. In the present study, the intersexes of Siphlonurus lacustris are totally males. This point slightly balances the gender of mayfly gynandromorphs.

From a biogeographic perspective, there are six intersexes collected from the Oriental realm (Zhejiang, Hunan, Hubei, Guizhou, and Fujian), one case in the transitional region between the Palearctic and Oriental realms (Anhui), and 14 cases from the Palearctic realm (Inner Mongolia and Heilongjiang) (Figure 10). This data, along with previous findings in China, suggests that the occurrence of mayfly gynandromorphism in China is similar to that in America, occurring from north to south without apparent geographical restrictions. Mayfly gynandromorphism appears to be a widespread and global phenomenon.

The causes of the gynandromorphism phenomenon in Ephemeroptera reportedly include the following three types: (1) XO karyotype; (2) parthenogenesis and polyspermy; and (3) parasite infection [4,5,6,18,19,20,21,22]. Yang et al. [4] implied that the similar external morphology of a male and female Choroterpes intersexes was caused by similar environmental factors. In this study, furthermore, the similar morphology and relatively rich cases of the Siphlonurus lacustris show that parasites may be a low possibility as a factor because they cannot cause so uniform cases. Our own hypothesis is that inorganic factors, like temperature, seem to play a major role in causing mayfly gynandromorphism. However, this is only a hypothesis; the real reason needs more studies to confirm.