Behavioral decisions are important mechanisms that can prevent interbreeding of species, and are critical to the evolutionary fate of natural hybrids. The project seeks to understand how these behavioral decisions affect the genetics of populations. Single nucleotide differences in DNA sequences between two species of neotropical stream fish will be used to study the genetics of hybrid populations in an area of Mexico where the two species hybridize in the wild. Since hybrid fish are sometimes difficult to visually distinguish from parent species, this molecular approach will enable investigators to identify fish as hybrid or parent species. Knowing the species identity of individuals will allow the investigators to examine behavioral decisions by determining who females mate with (mate choice decisions) and who individuals spend time with (social decisions), as well as whether there are differences in post-birth survival among hybrids and pure parentals in these livebearing fish. Preliminary results suggest that behavioral decisions may be more important in shaping the genetics of populations than is selection against hybrid fish. Further molecular work will allow more in-depth analyses of behavior and genetic patterns. This work will build and expand upon understanding the contributions of both pre-mating behavior (social decisions and mate choice decisions) and post-mating behavior (differential fertilization of eggs and natural selection) to the fate of natural hybrids, reproductive isolation of species and speciation. The project and investigators will accomplish outreach through the CICHAZ field station in Mexico via electronic media and mentoring of a Mexican student in molecular techniques in addition to continuing to foster internationally collaborative projects.
The project enabled us to conduct a variety of experiments in the Xiphophorus hybrid zones. The first outcome was showing that although hybridization likely occured as a result of reproductive isolation, each parental species appears to be behaviorally isolated within structured populations, suggesting that hybridization between parentals is not ongoing. Secondly, genotyping of females and embryos in structured and unstructured populations revealed that, indeed, pure parental females only mated with their own species and hybrids as no F1 embryos were observed. Additionally, genotyping embroys also demonstrated that there is no post-partuition selection against embryos as hybrids are found at equal frequencies at the embryo and adult stages of life. These two outcomes suggest that hybridization occured in the past, but still has lasting affects as introgression continues to occur due to mating between hybrids and pure parentals, thus maintaining gene flow between pure parentals despite lack new hybridization events. Next, morphological analyses of vertical bars confirmed showed signatures of this non-random mating in structured populations as these populations have significantly greater symmetry than both unstructured and parenal populations. Finally, the project directly helped collection of data on thermal tolerance and genetics in the hybrid zones. These data revealed that temperature plays a central role in maintaining species distributions and the structure of hybrid zones. All of these outcomes represent novel findings and contribute to increasing our understanding of the variables that maintain both hybrid zone structure and population structure within the hybrid zones. These processes that maintain hybrid zones and affect gene flow within them such as non-random mating and selection by the abiotic environment have important consequences for gene flow and the fate of hybrids and parentals. During the course of the work, five undergraduate students contributed time learning population genetic techniques in the lab at Texas A&M as a part of research credit hours, one of whom contributed sufficiently to be an author on a manuscript that is in preparation. The project also enabled continuing collaboration with a student, Christian Bautista-Hernandez, from the lab of Dr. Scott Monks at the Universidad Autonoma del Estado de Hidalgo, collecting data on parasite loads and SNP genotypes, which will be publishable with one more season of field work. Both PIs also presented findings of the research and paths in science to approximately 100 high school students at the local high school in Mexico where the field station is located. All findings have either been submitted for publication in international peer-reviewed journals or are in preparation for submission.
