CLONAL INHERITANCE OF A DIPLOID NUCLEAR GENOME BY A HYBRID FRESHWATER MINNOW (PHOXINUS EOS-NEOGAEUS, PISCES: CYPRINIDAE).

Hybrids between the minnows Phoxinus eos and Phoxinus neogaeus coexist with a population of P. eos in East Inlet Pond, Coos Co., New Hampshire. Chromosome counts and flow cytometric analysis of erythrocyte DNA indicate that these hybrids include diploids, triploids, and diploid-triploid mosaics. The mosaics have both diploid and triploid cells in their bodies, even within the same tissues. All three hybrid types are heterozygous at seven putative loci for which P. eos and P. neogaeus are fixed for different allozymes, indicating that the hybrids carry one eos and one neogaeus haploid genome. The diploid hybrids are therefore P. eos-neogaeus, whereas the triploids and mosaics are derived from P. eos-neogaeus but have an extra eos or neogaeus genome in all or some of their cells. Diploid, triploid, and mosaic hybrids accept tissue grafts from diploid hybrids, indicating that all individuals carry the identical eos-neogaeus diploid genome. Thus, one P. eos-neogaeus clone exists at East Inlet Pond. Grafts among the triploids and mosaics or from these individuals to diploid hybrids are rejected, indicating that the third genome is different in each triploid and mosaic individual. In this study, diploid and mosaic hybrids, carrying the clonal eos-neogaeus genome, were bred in the laboratory with males of P. eos or P. neogaeus. Both diploid and mosaic hybrids produced diploid, triploid, and mosaic offspring, revealing the source of the three hybrid types present at East Inlet Pond. These offspring accepted grafts from P. eos-neogaeus individuals, indicating that they all had inherited the identical eos-neogaeus genome. Most grafts among triploid and mosaic progeny, or from these individuals to their diploid broodmates, were rejected, indicating that the third genome was different in each triploid and mosaic (as was observed in the wild hybrids) and was contributed by sperm from males of P. eos or P. neogaeus. Diploid progeny are produced if sperm serves only to stimulate embryogenesis; triploid or mosaic progeny are produced if the sperm genome is incorporated. Although based on a mode of reproduction that by definition results in a genetically identical community of individuals, i.e., gynogenesis, reproduction in hybrid Phoxinus results in a variety of genetically distinct individuals by the incorporation of sperm into approximately 50% of the diploid ova produced.