BAKER, K. A.*; BREWER, A. J.; HANSEN, J. J.; STRAUMAN, M. A.; DAWLEY, R. M.; Ursinus College; Ursinus College; Ursinus College; Ursinus College; Ursinus College: Tissue-specific ratios of diploid and triploid cells in unisexual hybrids of the minnows Phoxinus eos and P. neogaeus.

Unisexual hybrids of the minnows Phoxinus eos and P. neogaeus include diploids, triploids, and, most unusually, diploid-triploid mosaics, individuals comprising a mixture of diploid and triploid cells. The clonal diploids arise by gynogenesis, sperm from males of P. eos triggering embryogenesis but not being incorporated into the embryo�s diploid eos-neogaeus genome. Triploids and mosaics arise when syngamy does occur and contain an eos-neogaeus-eos genome in all or some of their cells, respectively. We compared ratios of triploid (3N) to diploid (2N) cells in blood, liver, heart, and brain tissues of mosaic hybrids. In any particular tissue, 3N/2N ratios ranged from nearly 0 (virtually diploid) to nearly 1 (virtually triploid). Furthermore, within an individual fish 3N/2N ratios varied considerably, occasionally to the extent that one tissue was purely triploid and another purely diploid, and often (15/23 of all individuals) including at least a two-fold difference in ratio. In 18/23 individuals examined, the brain exhibited either the highest or the lowest 3N/2N ratio. Brain, of ectodermal origin, has a developmental history distinct from that of mesodermal heart and blood. That the 3N/2N ratio in brain differs from those of other tissues suggests that ploidy ratios are at least partly determined relatively early in development.