Transient hybridization, not homoploid hybrid speciation, between ancient and deeply divergent conifers

Premise of the study: Homoploid hybrid speciation is receiving growing attention due the increasing recognition of its role in speciation. We investigate if individuals intermediate in morphology between the two species of the conifer genus Athrotaxis represent a homoploid hybrid species, A. laxifolia, or are spontaneous F₁ hybrids.

Methods: A total of 1055 individuals of Athrotaxis cupressoides and A. selaginoides, morphologically intermediate individuals, and two putative hybrid swarms were sampled across the range of the genus and genotyped with 13 microsatellites. We used simulations to test the power of our data to identify the pure species, F₁s, F₂s, and backcross generations.

Key results: We found that Athrotaxis cupressoides and A. selaginoides are likely the most divergent congeneric conifers known, but the intermediates are F₁ hybrids, sharing one allele each from A. cupressoides and A. selaginoides at six loci with completely species specific alleles. The hybrid swarms contain wide genetic variation with stronger affinities to the locally dominant species, A. selaginoides and A. selaginoides backcrosses outnumbering A. cupressoides backcrosses. In addition, we observed evidence for isolated advanced generation backcrosses within the range of the pure species.

Conclusions: We conclude that, even though they can be large and long-lived, Athrotaxis hybrid swarms are on a trajectory of decline and will eventually be reabsorbed by the parental species. However, this process may take millennia and fossil evidence suggests that such events have occurred repeatedly since the early Quaternary. Given this timeline, our study highlights the many obstacles to homoploid hybrid speciation.