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 (␦ ϭ 0.68) in the more diverse population and somewhat lower (␦ ϭ 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.

The proximate determinants of sex ratio in C. elegans populations

“…Consistent with this assumption, we find no evidence that male reproductive success is frequency-dependent at the population densities used in this study (see Section 4). Inbreeding depression is likely to be an important factor in maintaining outcrossing in some taxa, such as the clam shrimp E. texana (Otto et al, 1993;Weeks & Zucker, 1999 ;Weeks et al, 2000). Despite its long history of selfing, inbreeding depression would still play a role in determining fitness in C. elegans following the occurrence of new deleterious recessive mutations.…”

“…Sex ratio in C. elegans was first treated in a model that utilized Fisher's concept of reproductive value (Fisher, 1930;Hedgecock, 1976). Stewart & Phillips (2002) extended to C. elegans the population genetic modelling approach taken to explore androdioecy in the selffertilizing clam shrimp Eulimnadia texana (Otto et al, 1993 ;Weeks & Zucker, 1999;Medland et al, 2000;Weeks et al, 2000). Another recent theoretical study developed a simple model of the influence of male reproductive ability and sex chromosome nondisjunction on sex ratio in C. elegans (Chasnov & Chow, 2002).…”

Life at low temperatures: A novel breeding‐system adjustment in a polar cladoceran

“…Species reproduction methods in this group (and sometimes even populations within species) range from pure gonochoristic sexuality through androdioecy (with various frequencies of males coexisting with hermaphrodites) to unisexuality. In these latter cases, breeding appears to be entirely by selffertilization (Sassaman 1995;Weeks et al 2000Weeks et al , 2005. Given the morphological and molecular evidence for the close relationship between cladocerans (together with Cyclestheria) and spinicaudatans (Olesen 2000;deWaard et al 2006;6-gene topology), it is possible that the adoption of self-fertilization in cladocerans could represent a resurrection of this breeding mode acquired from a common ancestor.…”

“…The latter breeding system is unknown in cladocerans but is common in two allied groups of branchiopod crustaceans, the notostracans and conchostracans (Sassaman 1995;Sassaman et al 1997;Weeks et al 2000). Because of this fact, a firm diagnosis of reproductive system for female-only populations requires either formal breeding studies or the survey of genotypic diversity in natural populations.…”

Chromosomes in sexual populations of Notostracan and Con-chostracan taxa (Crustacea, Branchiopoda)

“…In spite of their "unusual morphological conservatism" (Fisher 1990), Notostraca and also Conchostraca show an extensive variation in their reproductive biology with an array of highly diversified reproductive modes that adds many problems to their taxonomy. In particular, reproductive strategies of even a single taxon may range from gonochorism (sex ratio varying in different populations from equality to a female or a male bias) to self fertilizing hermaphroditism and parthenogenesis; further, Triops newberryi (Packard) (Notostraca) and Eulimnadia texana Packard (Conchostraca) (Sassaman 1991(Sassaman , 1995Sassaman et al 1997;Weeks et al 2000) are known as androdioecious animal systems (populations embody males in low frequencies and self-compatible hermaphrodites).…”