The observation that many closely related species differ in chromosome number and morphology has lead to the idea that chromosomal rearrangements (fusions, fissions, inversions, etc) may be contribute to reproductive isolation (the inability to interbreed). Recent evidence has linked certain types of chromosomal rearrangements (inversions) and the development of reproductive isolation. However, other types of common rearrangements, such as chromosomal fusions, have yet to be investigated in this capacity. The proposed studies will determine the importance of a chromosomal fusion to the development of reproductive isolation between two killifish in the genus Lucania. This will be accomplished using a three stage design. First a breeding design will create hybrids that vary in their possession of the fused chromosome. Second, these animals will be measured for several traits important in reproductive isolation. Third, the animals will be genotyped to determine if they possess the fused chromosome. This will allow for the detection of any relationship between reproductive isolation and the fused chromosome.
This is one of the first studies to empirically examine the effects of a chromosomal fusion on reproductive isolation between two recently diverged species. This project will impact the Lucania community through the development of new molecular markers as well as the creation of two linkage maps. Participation of undergraduates will continue to be a priority and will involve hands on behavioral and molecular experience. In addition, a student with disability will be invited to perform a related research project.
Many closely related species differ in chromosome number, size, and shape. This has lead to the idea that chromosomal rearrangements may be contribute to reproductive isolation (the inability to interbreed). Many studies have linked certain types of chromosomal rearrangements (inversions), adaptation to different environmental conditions, and the evolution of reproductive isolation. However, other types of common rearrangements, such as chromosomal fusions (where two chromosomes fuse together to make a larger fused chromosome) have yet to be investigated in this capacity. This study sought to determine the importance of a chromosomal fusion to the development of reproductive isolation between two killifish, the Bluefin Killifish (Lucania goodei) and the Rainwater Killifish (Lucania parva). Lucania goodei has 24 chromosome pairs, but L. parva only has 23 because a chromosomal fusion has occurred in this species. These two species also differ in salinity tolerance where L. parva is euryhaline, but L. goodei is a freshwater specialist. Evolutionary theory predicts that if a chromosomal rearrangement aided reproductive isolation then the genes for salinity tolerance and reproductive isolation should be located in the rearrangement. To investigate this, we used a breeding design that created hybrids that varied in their possession of the fused chromosome. These animals were measured for several traits important in reproductive isolation and then genotyped to determine if they possess the fused chromosome. Four main results emerge from this study. First, a chromosomal fusion has clearly occurred. Two linkage groups found separately in L. goodei occur together as the fused chromosome in L. parva. Second, the locus for sex determination occurs on the fused chromosome. Third, this same region of genome (with the sex determination locus) also plays a role in a number of interactions with other genes that contribute to salinity tolerance and reproductive isolation. However, this important region of the genome occurs on one distal end of the fused chromosome and is not shielded from recombination. Hence, the fused chromosome plays a critical role in speciation, but not due to the suppression of recombination.
