Sperm phenotype of admixed and ‘pure’ subspecies males in the long-tailed finch

Reproductive isolation is central to the speciation process. Recent research suggests that where heterospecific mating occurs, postmating prezygotic (PMPZ) barriers and hybrid infertility can act as powerful reproductive barriers. Two traits that may underlie PMPZ isolation and reduced hybrid fertility are sperm morphology and motility. The two subspecies of long-tailed finch (Poephila acuticauda acuticauda and P. a. hecki) offer an excellent opportunity to investigate mechanisms of reproductive isolation as their narrow hybrid zone in the wild suggests incomplete reproductive isolation, and they can be studied in captivity. In this study we examine sperm morphology and motility in experimentally created hybrid long-tailed finches and compare them to ‘pure’ males of each subspecies. We found that hybrid males had longer midpiece than P. a. hecki males and significantly longer flagellum length than both subspecies, which we discuss in the context of the expected genetic composition of these groups. However, we found no evidence that ‘hybrid’ males had low sperm velocity or proportion of motile sperm in vitro, or for a relationship between sperm morphology and velocity. We discuss the opportunity posed by this system to investigate the genomic basis of reproductive traits that could propel this research field forward.