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No abstract

Diploid-triploid mosaics are rarely found in vertebrates, and until now they were known to be common in only two vertebrate species complexes. Here we report that diploid-triploid mosaics are widespread among unisexual hybrids of the minnows Phoxinus eos and Phoxinus neogaeus, a complex already known to contain diploid and triploid forms. Using chromosome counts and flow cytometry, we show that the mosaics occur throughout the known range of the unisexuals and are abundant in many of these natural populations. The mosaics are highly heterogeneous, showing individual variation in the ratio of diploid to triploid cells, and as a group they appear to form a continuum between the pure diploid and triploid forms. Tissue-graft analysis shows that the third genome present in the triploid cells of a mosaic is expressed, because grafts made from the mosaics show an incidence of rejection intermediate between that of the diploid (clonal) and triploid (nonclonal) biotypes.

Using DNA-DNA hybridization, we have determined the degree of single-copy DNA (scDNA) divergence among eight species ofthe Drosophila obscura group. These include Old World and New World species as well as members of two subgroups. Contrary to classical systematics, members of the affinis subgroup are more closely related to American members of the obscura subgroup than are Old World species. The Old World species are not a monophyletic group. The degree of scDNA divergence among species is not necessarily correlated with morphology, chromosomal divergence, or ability to form hybrids. A unique pattern ofhybrid formation was found: species separated by a fiT m of6.5°C can form hybrids whereas species separated by a fiT m of2.5°C cannot. As with other groups of Drosophila, the obscura group has discrete parts of the genome evolving at very different rates. The slow evolving fraction of the nuclear genome is evolving at about the same rate as mitochondrial DNA. The additional scDNA divergence accompanying the step from partial reproductive isolation (between North Americanpseudoobscura and the isolated Bogota population) to full isolation is very small. The resolution of the technique was challenged by five closely related taxa with a maximum fiT m of2.5°C separating them; the taxa were unambiguously resolved and the "correct" phylogeny recovered. Finally, there is some indication that scDNA in the obscura group may be evolving considerably slower than in the melanogaster subgroup.

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.