Aclonal Reproduction by Polyploid Members of the Clonal Hybrid Species Phoxinus eos-neogaeus (Cyprinidae)

Abstract

Hybrids of the daces Phoxinus eos and P. neogaeus occur throughout the northern United States and southern Canada. Diploid hybrids and some diploid-triploid mosaic hybrids from East Inlet Pond, Coos County, New Hampshire, produce diploid all-identical ova of P. eos-neogaeus that develop into diploid young if sperm from either parental species serves only to stimulate development but triploid or diploid-triploid mosaic young if the sperm genome is actually incorporated. The hybrid population at East Inlet Pond appears to be perpetuated by this unique form of sperm-dependent, clonal reproduction. Here we report two additional forms of reproduction based on laboratory spawnings of two mosaic and two triploid hybrid females collected from East Inlet Pond, New Hampshire. The two triploids produced diploid offspring that were indistinguishable from P. eos on the basis of several external morphological traits. In addition, the offspring possessed allozymes characteristic of P. eos at all six diagnostic loci tested, and they had histocompatibility antigens and an allele of Gpi-B absent from their mothers. Conversely, the coiling of their digestive tracts varied between the morphology characteristic of P. neogaeus and that of P. eos. Collectively, these data suggest that the triploid females produced haploid ova containing exclusively or mostly chromosomes of P. eos that were fertilized by sperm of P. eos. The two mosaics produced offspring that carried the clonal genome of P. eosneogaeus, indicating that they produced diploid all-identical ova of P. eos-neogaeus like the diploid and mosaic hybrids we have previously reported. However, they also produced some offspring that did not carry the clonal genome, apparently from nonidentical ova of unknown ploidy level. Reproduction by the types of triploid and mosaic hybrids reported here may introduce greater variation into hybrid populations. These modes of reproduction may also provide a vehicle by which genes may be exchanged between the parental species, P. eos and P. neogaeus.