Bryology for the Twenty First Century
Genetic analysis of a hybrid zone in Mielichhoferia (Musci)
Shaw, AJ
Previous work demonstrated that the 'copper mosses' Mielichhoferia elongata and M. mielichhoferiana are genetically and morphologically distinct, but hybridize at five sites in Scandinavia and the United States where they grow in mixed populations. The present report describes genetic and morphological variation among 108 plants collected in a hybrid zone involving these two species at the Eureka Mine, San Juan County, Colorado. Allelic profiles at five polymorphic allozyme loci show that the relative proportions of M. elongata, M. mielichhoferiana, and recombinant gametophytes in the population are 19%, 58%, and 23%, respectively. The distribution of hybrid indices calculated from allozyme data suggest that genetically, recombinant gametophytes are skewed in the direction of the pure M. mielichhoferiana parental type. Morphological data are consistent with that interpretation; plants identified as recombinants based on allozyme data are intermediate between the parental species, but are closer (on average) to M. mielichhoferiana than to M. elongata. These observations suggest that primary recombinants derived from F1interspecific hybrids have backcrossed preferentially to M. mielichhoferiana. Alternatively, there may have been nonrandom mortality in the population, with M. mielichhoferiana types having a selective advantage. All three classes of gametophytes (M elongata, M. mielichhoferiana, recombinants) form apparently functional archegonia and antheridia. In the population as a whole there was significant linkage disequilibrium (D) among all pairs of allozyme loci, suggesting that despite extensive hybridization, the two species remain genetically distinct. In contrast, a complete absence of linkage disequilibrium in the recombinants is attributable to recombination among the unlinked allozyme loci following interspecific hybridization. Sporophytes sampled from the population appear to belong exclusively to gametophytes of M. elongata. A testable hypothesis to explain the genetic and morphological patterns observed in this study is proposed: hybridization in the population is asymmetric with regard to male and female parentage, with M. elongata acting as the predominant female parent and M. mielichhoferiana as the predominant male parent. This hypothesis leads to the prediction that recombinant gametophytes contain nuclear genes from both parents but have maternally inherited chloroplast genes predominantly or exclusively from M. elongata. This prediction can be tested using molecular markers from the chloroplast and nuclear genomes.
