Maintenance of Androdioecy in the Freshwater Shrimp, Eulimnadia Texana: Estimates of Inbreeding Depression in Two Populations

Abstract

Androdioecy is an uncommon form of reproduction in which males coexist with hermaphrodites. Androdioecy is thought to be difficult to evolve in species that regularly inbreed. The freshwater shrimp Eulimnadia texana has recently been described as both androdioecious and highly selfing and is thus anomalous. Inbreeding depression is one factor that may maintain males in these populations. Here we examine the extent of "late" inbreeding depression (after sexual maturity) in these clam shrimp using two tests: (1) comparing the fitness of shrimp varying in their levels of individual heterozygosity from two natural populations that differ in overall genetic diversity; and (2) specifically outcrossing and selfing shrimp from these same populations and comparing fitness of the resulting offspring. The effects of inbreeding differed within each population. In the more genetically diverse population, fecundity, size, and mortality were significantly reduced in inbred shrimp. In the less genetically diverse population, none of the fitness measures was significantly lowered in selfed shrimp. Combining estimates of early inbreeding depression from a previous study with current estimates of late inbreeding depression suggests that inbreeding depression is substantial (delta = 0.68) in the more diverse population and somewhat lower (delta = 0.50) in the less diverse population. However, given that males have higher mortality rates than hermaphrodites, neither estimate of inbreeding depression is large enough to account for the maintenance of males in either population by inbreeding depression alone. Thus, the stability of androdioecy in this system is likely only if hermaphrodites are unable to self-fertilize many of their own eggs when not mated to a male or if male mating success is generally high (or at least high when males are rare). Patterns of fitness responses in the two populations were consistent with the hypothesis that inbreeding depression is caused by partially recessive deleterious alleles, although a formal test of this hypothesis still needs to be conducted.

DNA methylation in adults and during development of the self‐fertilizing mangrove rivulus, Kryptolebias marmoratus

“…Consequently, a deep understanding of the role of epigenetic variation in evolution can only be achieved in individuals that are genetically identical but exhibit a range of heritable phenotypes in nature (Heard & Martienssen, 2014 (Costa, 2011;Harrington, 1961;Tatarenkov, Lima, Taylor, & Avise, 2009). In nature, hermaphrodites coexist with a low proportion of males (mostly fewer than 5%) Mackiewicz, Tatarenkov, Taylor, Turner, & Avise, 2006;Mackiewicz, Tatarenkov, Perry, et al, 2006), but no females, which constitutes a rare androdioecious mixed reproductive system Weeks, Crosser, Bennett, Gray, & Zucker, 2000). Given the ability of hermaphrodites to lay unfertilized eggs, outcrossing with males is possible but less frequent than selfing (Mackiewicz, Tatarenkov, Taylor, et al, 2006;Mackiewicz, Tatarenkov, Perry, et al, 2006).…”

Production of intersexes and the evolution of androdioecy in the clam shrimpEulimnadia texana(Crustacea, Branchiopoda, Spinicaudata)

“…Since that early work, experimental results suggest that sex is more realistically determined by either a large linkage group or possibly by sex chromosomes with limited crossing over (Weeks et al, 1999;Weeks et al, 2000;Weeks et al, 2001). Genetic evidence has revealed that three allozyme loci are physically linked to the sex determining region, and that crossing over among these loci is ~1% (Weeks et al, 1999).…”

Maintenance of androdioecy in the freshwater clam shrimp <i>Eulimnadia texana</i>: longevity of males relative to hermaphrodites

“…When we combine our estimates of relative male viability (1 -σ) with previous estimates of inbreeding depression (δ) (Weeks et al 1999(Weeks et al , 2000b, we can begin to address the stability of E. texana's mixed mating system using Otto et al's (1993) model. The estimates of relative male viability reported here were found to range from 0.80 to 0.87 for JT4 and from 0.67 to 0.94 for WAL ( Table 2).…”

“…The estimates of relative male viability reported here were found to range from 0.80 to 0.87 for JT4 and from 0.67 to 0.94 for WAL ( Table 2). The lifetime cumulative inbreeding depression (δ) in these populations has been estimated as 0.50 for JT4 and 0.68 for WAL (Weeks et al 2000b). Plugging these low (0.80 and 0.67) and high (0.87 and 0.94) estimates of relative male viability for JT4 and WAL, respectively, into Otto et al's (1993) model, we suggest that for the mixed mating systems found in these two populations to be evolutionarily stable, the following conditions must be met: low: α > 1.25β (JT4) and α > 0.96β (WAL) high: α > 1.15β (JT4) and α > 0.68β (WAL)…”

“…Otto et al's (1993) model proposes that if the proportion of eggs fertilized by males (α) multiplied by the viability of males relative to hermaphrodites (1 -σ) (i.e., the relative fitness of outcrossing) is greater than 2 times the proportion of eggs not fertilized by males but fertilized by hermaphrodites (β) multiplied by the relative inbreeding depression suffered by selfed offspring (1 -δ) (i.e., the relative fitness of selfing), then males and both types of hermaphrodites will be maintained in the population. However, so little is known about the reproductive biology and life history of this species that estimates of these parameters are lacking (but see Weeks et al 1999Weeks et al , 2000b and several assumptions of Otto et al's (1993) model remain untested.…”