Reproductive isolation between diploid and tetraploid individuals in mixed-cytotype populations of Lycium australe

Whole-genome duplication is considered a major mechanism of sympatric speciation due to the creation of strong and instantaneous reproductive barriers. Although postzygotic reproductive isolation between diploids and polyploids is often expected, the extent of reproductive incompatibility must be empirically determined and compared to patterns of genetic isolation to fully characterize the reproductive dynamics between cytotypes.

We investigated reproductive compatibility between diploid and tetraploid Lycium australe in two mixed-cytotype populations using (1) controlled crossing experiments to evaluate fruit and seed production and (2) germination trials to test seed viability following homoploid and heteroploid crosses. We contrast these experiments with a single-nucleotide polymorphism (SNP) data set to measure genetic isolation between cytotypes and explore whether cytotype or population origin better explains patterns of genetic variation. Finally, we explore mating patterns using the observed germination rates of naturally produced seeds in each population.

Although homoploid and heteroploid crosses resulted in similar fruit and seed production, reproductive isolation between co-occurring diploids and tetraploids was nearly complete, due to low seed viability following heteroploid crosses. Of 191,182 total SNPs, 21,679 were present in ≥90% of individuals and replicate runs using unlinked SNPs revealed strong clustering by cytotype and differentiation of tetraploids based on population origin.

As often reported, diploid and tetraploid L. australe experience strong postzygotic isolation via hybrid seed inviability. Consistent with this result, cytotype explained a greater amount of variation in the SNP data set than population origin, despite some evidence of historical introgression.