Quantification of Reproductive Isolating Barriers Between Two Naturally Hybridizing Killifish Species
Understanding the relative importance of various reproductive barriers to the early stages of speciation is an essential question in evolutionary biology. The closely related killifishes Fundulus heteroclitus and F. grandis occasionally hybridize in a small region in coastal Northeastern Florida showing that while barriers to reproduction exist, they are incomplete. The objective of this study was to elucidate barriers to reproduction between F. heteroclitus and F. grandis in the lab, as well as to quantify their strengths and relative contributions to reproductive isolation. Pre-zygotic (mating and fertilization) and post-zygotic (hatching) barriers were investigated by performing a variety of choice and no-choice laboratory mating experiments. Under no-choice conditions, barriers to mating had the greatest influence on hybrid production in F. grandis, whereas hatching barriers contributed to the majority of reproductive isolation in F. heteroclitus. Under choice conditions, however, pre-zygotic barriers had the greatest influence on hybrid production in both species. The total reproductive isolation that was observed in females of each species was stronger in F. heteroclitus than in F. grandis, and was nearly complete in F. heteroclitus females under choice conditions and was of moderate strength in F. grandis females. These results reveal an asymmetry in the potential gene flow between these two species, with F. grandis being more likely to hybridize than F. heteroclitus in the absence of environmental influences. No-choice backcrosses were also conducted and showed that at least some F1 hybrids are fertile. The observation that pre-zygotic barriers tend to be stronger than post-zygotic barriers in the early stages of speciation is consistent with similar studies in other organisms.
KeywordsReproductive isolation Speciation Hybridization Fundulus
We would like to thank the Lerner-Gray Memorial Fund of the American Museum of Natural History, the UNF Graduate School, and the UNF Coastal Biology Program for providing funding for this research. We thank Carlos Barbas, Jennifer Raabe, Victor Senf, Veronica Logue and Leigh Jordan for their help with collecting and caring for animals. Additionally, we would like to thank Dr. Kelly Smith and Dr. Eric Johnson for their comments on previous versions of this work and to Dr. Elena Buzaianu for statistical suggestions.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Coyne, J. A., & Orr, A. H. (2004). Speciation. Sunderland: Sinauer Associates, Inc.Google Scholar
- Dobzhansky, T. (1937). Genetics and the origin of species. New York: Columbia University Press.Google Scholar
- Foster, N. R. (1967). Trends in the evolution of reproductive behavior in killifishes. In: F. M. Bayer et al., (Eds.), Proceedings of the international conference on tropical oceanography (pp. 549–566). Miami, FL: University of Miami Institute of Marine Sciences.Google Scholar
- Howard, D. J. (1993). Reinforcement: Origin, dynamics and fate of an evolutionary hypothesis. In R. G. Harrison (Ed.), Hybrid zones and the evolutionary process (pp. 46–69). New York: Oxford University Press.Google Scholar
- Jordan, D., & Evermann, B. (1898). The fishes of North and Middle America: A descriptive catalogue of the species of fish-like vertebrates found in the waters of North America, north of the Isthmus of Panama, 47th ed. US Government Printing Office, Washington, District of Columbia.Google Scholar
- Mayr, E. (1942). Systematics and the origin of species, from the viewpoint of a zoologist. Cambridge: Harvard University Press.Google Scholar
- Mayr, E. (2000). The biological species concept. In Q. D. Wheeler & R. Meier (Eds.), Species concepts and phylogenetic theory: A debate (pp. 17–29). New York: Columbia University Press.Google Scholar
- Muller, H. J. (1942). Isolating mechanisms, evolution, and temperature. Biology Symposium, 6, 71–125.Google Scholar