Abstract

Etheostoma is a genus of North American darter fish whose species have similar habitats and breeding seasons, yet hybridization is rare. Behavioral barriers have been demonstrated to play a key role in maintaining species boundaries. Further, conspecific (same species) sperm precedence has also been observed when the gametes of two different species come into contact. In this study, we investigated if physical characteristics of sperm could be a mechanism for the lower fertilization success of heterospecific (different species) males when eggs are simultaneously exposed to conspecific and heterospecific sperm. We chose to examine the sperm of two closely related species, E. zonale and E. barrenense. Using toluidine blue and immunofluorescent labeling methods, we compared head diameter and tail length of sperm cells between the two species. We found that head diameter was significantly larger for E. barrenense sperm compared to E. zonale. This difference in cell morphology may point to a physical mechanism underlying conspecific sperm precedence in Etheostoma. Our results are the first to describe a morphological difference in sperm between species in this genus and provide initial evidence for the role of sperm morphology in prezygotic reproductive isolation.

Highlights

  • Etheostoma, commonly known as darter fish, provide an excellent system to study the evolution of reproductive isolation (Williams& Mendelson, 2010)

  • E. zonale stained with toluidine blue had an average head diameter of

  • The average nuclear diameter for E. zonale samples stained with immunofluorescent labeling were 2.04 (± 0.09) μm and tail lengths of 26.18

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Summary

Introduction

Isolating mechanisms in these fish are intriguing, since they live in similar habitats, have similar breeding seasons, and fertilize externally. Multiple forms of reproductive isolation must be at work to prevent these species from hybridizing, given that there are ample opportunities for gametes to mix. It has been determined that behavioral pre-mating isolation is very strong and nearly complete in these fish (Mendelson et al., 2007). This does not address what prevents hybridizaHoward Community College, 10901 Little Patuxent Pkwy, Columbia, MD 21044. Barriers to hybridization that occur after mating are referred to as forms of post-mating prezygotic reproductive isolation

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