5.1. Selection at early life history stages
As discussed above, introgression occurs largely from I. fulva into I. brevicaulis and I. hexagona in both natural and experimental hybrid populations. Some of these results also allow a determination of possible reproductive barriers that contribute to this asymmetry. Gamete (or pollen) competition was presented as one of the prezygotic barriers underlying this asymmetry. There is also evidence that postzygotic barriers contribute to the directionality of introgression. In particular, viability selection disfavors introgressed genotypes towards I. fulva, but favors those towards I. brevicaulis.
Figures 1 and 2 and
Table 1 illustrate the effects of natural selection, at the seedling establishment stage, against certain hybrid genotypes in both natural and experimental hybrid populations. For example, there are numerous
I. fulva-like introgressed genotypes present in the seeds sampled from the natural hybrid zone that are not present in the adult iris plants (e.g. those with genotypic scores of 2-4;
Figure 1). In contrast, there are adult plants in all of the
I. brevicaulis-like introgressed categories (
i.e., 5-8). This indicates viability selection
against introgressed
I. fulva genotypes, but likely
for certain introgressed
I. brevicaulis genotypes [
6,
20]. Similarly,
Figure 2 reflects selection that disfavors hybrids containing a higher frequency of introgressed
I. brevicaulis alleles, but favors hybrids with a higher proportion of
I. fulva alleles [
23]. Finally, this lack of penetration of
I. brevicaulis alleles can also be seen in the data presented in
Table 1. In this instance, data from a single locus (as with those given in
Figure 2) indicates that
I. fulva alleles are incorporated into adult
I. brevicaulis-like plants, but
I. brevicaulis alleles are almost completely excluded from plants that are
I. fulva-like [
20].
5.2. Selection at later life history stages
From the above, we see that there is evidence consistent with selection-generated asymmetry in introgression due to differential viability at early life history stages in Louisiana Irises. Similarly, a number of analyses have detected asymmetry in survivorship at latter stages of plant development. For example, the segregation distortion illustrated by
Figure 3 is due to differential survivorship of adult plants maintained in the greenhouse. Thus, even under what is assumed to be highly favorable environmental conditions (
i.e., the greenhouse), asymmetric introgression from
I. fulva into
I. brevicaulis was detected [
24].
The pattern of survivorship under natural conditions has also been studied by transplanting the same genotypes maintained in the greenhouse [
24] into field conditions in southern Louisiana.
Table 2 contains the observed survivorship frequencies for
I. fulva,
I. brevicaulis and introgressed genotypes of both species subsequent to a severe (water depth of several feet) and extended (ca. four month) natural flooding event [
31]. We detected the following, hierarchical, survivorship values:
I. fulva > Introgressed
I. fulva > Introgressed
I. brevicaulis >
I. brevicaulis. This pattern of survivorship is consistent with previous observations suggesting greater tolerance to root/rhizome submersion by
I. fulva relative to
I. brevicaulis [
18].
Table 2.
Survivorship frequencies for
I. brevicaulis,
I. fulva and introgressed genotypes of these two species. Survivorship estimates were derived after a severe flooding episode [
31].
Table 2.
Survivorship frequencies for I. brevicaulis, I. fulva and introgressed genotypes of these two species. Survivorship estimates were derived after a severe flooding episode [31].
Class | Alive | Dead | % Survival |
---|
I. brevicaulis | 0 | 13 | 0 |
Introgressed I. brevicaulis | 23 | 393 | 0.055 |
Introgressed I. fulva | 33 | 325 | 0.092 |
I. fulva | 3 | 8 | 0.273 |
In regard to asymmetric introgression, it might be argued that the above pattern (of greater survivorship of Introgressed
I. fulva genotypes relative to Introgressed
I. brevicaulis genotypes) would facilitate greater levels of introgression into
I. fulva. However, the data from both
Table 2 and
Table 3 argue against this inference. Instead, it is apparent that
I. fulva alleles often (but, not always, see [
31]) provide the basis for higher survivorship. First, the surviving “Introgressed
I. brevicaulis” genotypes contained a significantly higher number of positively selected
I. fulva alleles than
I. brevicaulis alleles (
Table 3, [
31]). Second,
I. fulva and “Introgressed
I. fulva” hybrids survived at the highest frequencies (
Table 2), reflecting the selective advantage of this species’ genetic background in a flooded environment. These field surveys will also allow an estimate of temporal fluctuations in the direction and/or strength of selection (e.g. as expected if drier environmental conditions, favoring
I. brevicaulis, occur).
Table 3.
The number of
I. fulva and
I. brevicaulis alleles inferred to be selectively favored in “Introgressed
I. brevicaulis” plants that survived the flooding episode (see
Table 2; [
31]).
Table 3.
The number of I. fulva and I. brevicaulis alleles inferred to be selectively favored in “Introgressed I. brevicaulis” plants that survived the flooding episode (see Table 2; [31]).
| Introgressed I. brevicaulis |
---|
Alleles Favored | I. fulva | I. brevicaulis |
Number of Alleles Favored | 101 | 41 |