Instantaneous reproductive isolation in polyploid treefrogs: Mechanisms and consequences

Abstract

Speciation by polyploidy—duplication of chromosome sets—commonly occurs in plants and happened twice in the early evolutionary history of vertebrates. The gray treefrog (Hyla versicolor) is a recently evolved tetraploid species found in the eastern United States, where it often occurs with a cryptic diploid species (Hyla chrysoscelis). Assortative mating by ploidy is favored by selection because of triploid hybrid sterility and is based solely on selective phonotaxis of females to the species (ploidy)-specific calls of males. The tetraploid species has arisen multiple times independently; the calls and preferences of frogs in these lineages are nearly the same. Autotriploids of the diploid species created in the laboratory show parallel shifts in both the key call-properties of males and female preferences in the direction of the wild-type tetraploid species. These shifts are probably caused by changes in cell size (larger in tetraploids) and number (fewer in tetraploids) rather than the difference in gene dosage. The magnitude of changes expected in autotetraploids would be sufficient for instant pre-mating isolation of diploids and recently derived tetraploids, hence promoting instantaneous speciation, which would then be reinforced by selection against the production of infertile hybrids.