Analysis of potential factors allowing coexistence in a sexual/asexual minnow complex

Abstract

The northern redbelly dace (Chrosomus eos) and the finescale dace (C. neogaeus) have hybridized to produce an all-female, asexual hybrid (C. eos-neogaeus) that reproduces by sperm-limited parthenogenesis (gynogenesis). However, in this system, gynogenesis is not 100 % efficient; triploid females are occasionally formed which reproduce as sexuals, producing nuclear males and females of the paternal species (generally C. eos). Thus, the asexual lineage continually produces occasional males that can serve as a sperm source. Because (almost) all hybrid offspring are females, the hybrid population has the potential to grow more quickly and even outcompete the sexuals, thus eliminating their own sperm source. The current research uses behavioral testing, ovarian analyses, and modeling to examine three hypotheses for the maintenance of the sexual/asexual complex: male discrimination against hybrid females, fecundity differences between sexual and asexual females, and production of nuclear male sexuals from the asexual lineage. Results suggest that males do not discriminate against asexual females, and that both sexual and asexual females have similar fecundities, eliminating these hypotheses as potential coexistence mechanisms. However, computer simulations of population growth support the hypothesis that occasional triploidy within the hybrid population supplies enough breeding males to maintain the sexual/asexual complex.

The effects of allospecific mitochondrial genome on the fitness of northern redbelly dace (Chrosomus eos)

“…These empirical results strongly contrast with the expectations of Barron et al. 's theoretical model (). Under their model, an abundance of hybrids allowing the formation of cybrids as low as 5% is expected to generate high amount of cybrids, leading the sympatric wild type to extinction (Barron et al., ).…”

“…Therefore, de novo production of cybrids is not only dependent on the presence of hybrids, as proposed by Barron et al. (), but also other factors, such as genetic, ecological, and environmental conditions, may hinder the efficiency of cybrid production.…”

“…'s theoretical model (). Under their model, an abundance of hybrids allowing the formation of cybrids as low as 5% is expected to generate high amount of cybrids, leading the sympatric wild type to extinction (Barron et al., ). In the current study, wild‐type C. eos and hybrids are found together in 13 different sites but only seven of them harbored locally produced cybrids.…”

“…However, de novo production of cybrids represents an additional input of individuals that can demographically favor cybrids. On the one hand, cybrids are expected to exclude wild types if they display fitness similar or higher to that of wild types, as theoretically demonstrated by Barron, Lawson, and Jensen (2016). On the other hand, a lower fitness of cybrids can be demographically compensated by de novo production of individuals so that both cybrids and wild types can coexist in sympatry.…”

Imitating the cost of males: A hypothesis for coexistence of all‐female sperm‐dependent species and their sexual host

“…Indeed, according to Barron et al. 's () theoretical model, the systematic production of cybrids by triploid individuals, even if the proportion of triploid is as less as 5%, would result in the total replacement of the ancestral sexual C. eos populations. However, previous studies reported the presence of C. eos in sympatry with gynogenetic and cybrid individuals in various natural populations throughout the C. eos‐neogaeus distribution (Angers & Schlosser, ; Mee & Taylor, ; Vergilino et al., ).…”