Taxonomic and populational studies of the thallose liverworts Riccia subgenus Riccia

Abstract

Starch-gel electrophoresis and karyology were employed to establish more precise species circumscriptions in the taxonomically troublesome thallose liverwort Riccia L. subgenus Riccia and to survey variation among natural populations of R. dictyospora Howe. Significant observations include the following: (1) Little karyotypic differentiation was apparent among the 12 species examined. With the exceptions of R. mcallisteri Howe (n = 16) and R. nigrella DC. (n = 8?), n = 8 (7 + 1 m) in all species. Although relative chromosome lengths were similar in all haploid species, a large range in absolute chromosome length was observed among species. (2) Electrophoretic assay of populations representing 15 morphological species from the United States revealed species-specific banding patterns for the isozymes of peroxidase and malate dehydrogenase. (3) Electrophoretic phenotypes consistently permitted identification of material not easily assignable to species by morphology alone. (4) Distinctive electrophoretic phenotypes support the specific recognition of R. mcallisteri and R. albida Sull., both of previously questioned validity . A lack of distinction between the electrophoretic phenotypes of R. hirta (Aust.) Underw. and R. eldeeniae Jacobs suggests that the two names may be synonyms. (5) The distinct electrophoretic phenotypes of R. mcallisteri indicate that n = 16 represents autodiploidy in this species. (6) A combination of morphological and electrophoretic evidence indicates the discovery of two Riccia species that are new to the United States and, as yet, unidentified. (7) At Enchanted Rock State Natural Area, Texas, the geographical distribution of the allozymes of malate dehydrogenase and isocitrate dehydrogenase in R. dictyospora suggests that gene flow can be effectively impeded by distances (along contour lines) of 1 km or less. (8) A survey of eight enzyme loci in R. dictyospora detected three distinct sets of genotypes among 38 sampling localities representing the central, southern, and southeastern United States. The apparent in ability of linkage to account for these genotypic sets and the frequent coexistence at individual collection localities of representatives of different genotypic sets suggest that R. dictyospora maybe a composite of several sibling species.