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Enzyme Variability and Modes of Evolution in Bommeria (Pteridaceae)

1985; American Society of Plant Taxonomists; Volume: 10; Issue: 1 Linguagem: Inglês

10.2307/2418438

1548-2324

Christopher H. Haufler,

Plant Diversity and Evolution

As is true of other plants inhabiting xeric environments, desert ferns may be mor- phologically similar because of convergent evolution. Since phylogenetically important reproduc- tive features in these plants may be affected by vegetative modifications, electrophoretically de- tectable variability in 11 enzymes coding for 13 loci has been analyzed to provide an expanded, comparative data base for defining systematic relationships and population structure in the four species of Bommeria. Contrary to the ad hoc assumption that because Bommeria species have relatively high chromosome numbers they should be polyploid, enzyme studies suggest that all species are genetic diploids. These studies also demonstrate that genetic variability is partitioned among the individuals within populations and that the sexual species are outcrossing. Analyses of allelic frequencies indicate that the four species are genetically more distinct from one another than are angiosperm congeners. Bommeria subpaleacea and B. ehrenbergiana emerge as the most similar taxa with B. pedata and B. hispida being more distant. The enzyme data, combined with other characters, support the hypotheses that the mountains of west-central Mexico are the center of origin of Bommeria and that the triploid apomict B. pedata was not derived through hybridization among extant sexual species. Although the corroborative morphological, biogeographical, and enzymatic data present a reasonable phylogenetic hypothesis, the extraordinary genetic distance among these congeners suggests that Bommeria may be polyphyletic, composed of species that are grouped together because of superficial resemblance. Evolution in plants of warm deserts has been characterized by considerable vegetative and physiological convergence (Orians and Solbrig 1977). In assessing characters for phylogenetic analysis of these plants, it is necessary to con- sider carefully the role that the environment may have played in directing morphological modifications. Among angiosperms, xero- morphic, perennial members of the Cactaceae, Euphorbiaceae, and Asclepiadaceae share many vegetative features but may be unambiguously separated into distinct evolutionary lines based on floral characters. Since flowers are relatively ephemeral, their features are less affected by environmental pressures and can be used as reasonably accurate indicators of phylogenetic relationships. Among the ferns, there is a suite of genera commonly associated with xeric habitats. These share many of the features considered to be adaptations for survival in environments hav- ing frequent droughts, low humidity, and high insolation. Plants in genera such as Cheilanthes, Notholaena, Pellaea, and Bommeria are small, have highly dissected leaves, and are often clothed