The Accumulation of Reproductive Barriers During Speciation: Postmating Barriers in Two Behaviorally Isolated Species of Darters (Percidae: Etheostoma)

Abstract

Identifying the manner in which reproductive barriers accumulate during lineage divergence is central to establishing general principles of species formation. One outstanding question is which isolating mechanisms form the first complete barrier to gene flow in a given lineage or under a particular set of conditions. To identify these initial reproductive barriers requires examining lineages in very early stages of divergence, before multiple reproductive barriers have evolved to completion. We quantified the strength of three postmating barriers in a pair of darter species and compared these estimates to each other and to the strength of behavioral isolation (BI) reported in a previous study. Results reveal no evidence of gametic incompatibility but intermediate levels of conspecific sperm precedence and hybrid inviability. As BI is nearly complete, our analysis comparing the strength of multiple reproductive barriers implicates the evolution of mate choice as central to both the origin and maintenance of these species. Further examination of ecological isolation and hybrid sterility is necessary to determine the role of these barriers in darter speciation.

Fucosidases of Sperm and Milt in Darters (Percidae: Etheostomatini)

“…Sperm concentrations (sperm/mL) and motility were determined microscopically using Cell-VU® counting chambers. Preparations like these, including some of those studied here, are fully capable of fertilizing fresh darter oocytes [33].…”

“…In addition, multiple species of darters may co-occur in a given habitat, making reproductive barriers of primary importance in maintaining species boundaries. For example, Mendelson et al [33] demonstrated that sperm of Etheostoma hopkinsi will outcompete those of E. luteovinctum for fertilization of conspecific (same-species) eggs. Such conspecific sperm precedence represents a potentially important barrier to genetic exchange between species (e.g.…”

Caribbean acroporid coral hybrids are viable across life history stages

“…The relative contribution of isolating barriers may differ; therefore, multiple postzygotic mechanisms across life history stages need to be assessed in order to determine the collective strength of reproductive isolation (Ramsey et al 2003, Mendelson et al 2007. Caribbean acroporids have a complex life cycle which includes external fertilization and a pelagic larval stage.…”

Chromosome inheritance and reproductive barriers in backcrosses between two hybridizing Viviparus snail species

“…Heterospecific sperm precedence may result from sexual conflict. Through sexually antagonistic coevolution, females may have evolved strong resistance to conspecific but not to heterospecific sperm, leading to increased fertilization success of heterospecific sperm after interspecific matings (Parker & Partridge, 1998;Martin & Hosken, 2004;Arnqvist & Rowe, 2005;Mendelson, Imhoff & Venditti, 2007). Although the mechanism for the observed sperm precedence in backcrosses with V. contectus females is not yet known, it may be that female-mediated processes are involved.…”