Persistent high levels of tetraploidy in salamanders of the Ambystoma jeffersonianum complex

Almost all unisexual members of the Arabystoma jeffersonianum complex of salamanders are triploid, although occasional tetraploid individuals have been observed in a few populations. A population characterized by a high percentage of tetraploids (7-27%) exists at Kickapoo State Park, Vermilion Co, Illinois. This high frequency of tetraploids has been maintained over a period of 12 years. Tetraploids in this population were originally formed by fertilization of unreduced A. platineum ova by A. texanum sperm. Tetraploids at Kickapoo State Park are, however, capable of obtaining sperm from the sexual host, Ambystoma texanum, and producing tetraploid offspring. Because tetraploids can be produced two ways, they might be expected to out-reproduce and replace triploids. However, these tetraploids also produce abundant pentaploid offspring as a result of fertilization of their ova by A. texanum sperm. A high incidence of physical abnormalities among pentaploid larvae and their scarcity as breeding adults suggests that pentaploids have reduced viability. Production of pentaploid larvae may thus be the selective mechanism that prevents tetraploids from replacing triploids in this population. Higher than normal water temperature may be causing increased fertilization rates in this population; this would account for its higher frequency of tetraploids compared to other populations, as well as for the high number of pentaploid larvae that tetraploids produce.