RESUMO
Since tropical trees often have long generation times and relatively small reproductive populations, breeding systems and genetic variation are important for population viability and have consequences for conservation. Miconia albicans is an obligate, diplosporous, apomictic species widespread in the Brazilian Cerrado, the savanna areas in central Brazil and elsewhere in the Neotropics. The genetic variability would be, theoretically, low within these male-sterile and possibly clonal populations, although some variation would be expected due to recombination during restitutional meiosis. We used ISSR markers to assess genetic diversity of M. albicans and to compare with other tropical trees, including invasive species of Melastomataceae. A total of 120 individuals from six populations were analysed using ten ISSR primers, which produced 153 fully reproducible fragments. The populations of M. albicans presented mean Shannon's information index (I) of 0.244 and expected heterozygosity (He ) of 0.168. Only two pairs of apparently clonal trees were identified, and genetic diversity was relatively high. A hierarchical amova for all ISSR datasets showed that 74% of the variance was found among populations, while only 26% of the variance was found within populations of this species. Multivariate and Bayesian analyses indicated marked separation between the studied populations. The genetic diversity generated by restitutional meiosis, polyploidy and possibly other genome changes may explain the morpho-physiological plasticity and the ability of these plants to differentiate and occupy such a wide territory and different environmental conditions. Producing enormous amounts of bird-dispersed fruits, M. albicans possess weedy potential that may rival other Melastomataceae alien invaders.
Assuntos
Melastomataceae/genética , Árvores/genética , Brasil , DNA de Plantas/genética , Marcadores Genéticos/genética , Variação Genética/genética , Repetições de Microssatélites/genética , Filogenia , Reação em Cadeia da Polimerase , Clima TropicalRESUMO
We conducted an isozyme study in 22 populations of five Pleurothallis (Orchidaceae) species (12 loci in nine enzymatic systems). The genetic variability in all populations is surprisingly high (P = 58-83%, A = 2.1-3.8, H(e) = 0.25-0.43) in spite of the fact that the five species are pollinated by small flies whose behavior enables self-pollination. We suggest that self-incompatibility, inbreeding depression, and mechanical barriers that prevent self-pollination in these species are responsible for the maintainance of the high genetic variability. These traits are uncommon in Orchidaceae, but have been observed in these and some other species pollinated by flies or other pollinators with behavior that facilitates self-pollination. The genetic similarity among conspecific populations is also high for species with very short-range flying pollinators. Only one population of P. teres presented values of genetic similarity lower than usually observed in allopatric conspecific populations. Morphology, however, does not support its segregation as a new taxon. All species can be recognized by their enzymatic patterns, and the results agree with recently proposed taxonomic realignments. Conversely, the supposed affinities among these species based on floral morphology are not supported, and we hypothesize that it may be due to convergence in species with similar pollinators.
