Diversidade genetica, taxa de cruzamento e estrutura espacial dos genotipos fissilis Vell. (meliaceae)

AUTOR(ES)
DATA DE PUBLICAÇÃO

1996

RESUMO

A natural population of Cedrela fissilis Vell. (Meliaeeae) was surveyed with the aim of estimating the genetic diversity and the outerossing rate of arare tree species, that is, a species with low populational density. The population site was at the Fazenda Intervales belonging to the "Fundação para a Conservação e Produção Florestal do Estado de São Paulo", located at Sete Barras - São Paulo State. In the study area, Cedrelafissilis is arare species, weere 34 adult trees occur in an area of 270 ha (mean density of one tree for 8 ha). To estimate the genetic parameters, the technique of isozyme eleetrophoresis in horizontal straeh gels was applied. Thirty-four adult trees and 5 families with 30 individual each, formed from seeds collected directly from 5 trees of the population were analized. Seven enzymatie systems (6-PGDH, PGI, G2DH, IDH, MDH, PO), resulting in 13 genetic loci, were utilized.For adult trees, the parameters was: mean heterozigosity = 0.222, heterozigosity for the polimorphic loci = 0.288, percentage of polimorphi lo i = 76.9% and mean number of alleles per loeus = 2.31. For families, the parameters was: mean heterozigosity = 0.193, heterozigosity for the polimorphie loei = 0.228, percentage of polimorphi loei = 84.6% and mean number of alleles per loeus = 2.38. The adult population was in Hardy-Weinberg equilibrium, but the families differed significantly from the equilibrium expectations. The mean apparent outerossing rat~ (ta) estimated from the coefficient of inbreeding was 0.85. The multiloeus outerossing rate (tm) was 0.92, but the individual outcrossing rates for each tree varied from 0.62 to 1.08. The mean single outcrossing rates (ts) was 0.83, therefor lower than tm, but did not differ significantly from it. All mother trees were fertilized by an homogeneous pollen pool, indicating random outcrossing. Utilizing alIeles exclusives to the families, the minimum distance of pollen flow (distance between the mother tree and the border of the surveyed area) was estimated and reached 950 m. To study the spatial genetic structure, spatial autocorrelation analysis through the I index of Moran was utilized. The small number of significant values showed that spatial distribution of genotypes is random. The results of this research show that the population of Cedrela fissilis, in spite of having a low density of adult trees, exhibits a high outcrossing rate, extensive polIen flow, random outcrossing and random spatial distribution of genotypes. Therefore, the presumptions that rare species exhibit limited gene flow by polen, show self-fertilization and biparental breeding, and have genetic structured populations do not apply to this population of Cedrela fissilis. The main conclusions of this study are: 1. :rhe genetic diversity showed by this population ofCedrelafissilis (H=O.222) is on the average for the populations of tropical tree species already studied. 2. Cedrelafissilis is preferably alogamous (tm=0.92), but can show variations among trees in the outcrossing rate (tindividual between 0.62 and 1.08). 3. The gene flow by pollen in Cedrelafissilis can reach a considerable distance (over 950m) agreeing with the low density of trees. 4. The population of Cedrela fissilis does not show genetic structure, suggesting that gene flow is sufficiently wide to prevent genetic structuring due to genetic drift and/or local selection. 5. The results for gene flow by pollen, breeding system and spatial distribution of genotypes agree amongst each other and are coherent with the low population density

ASSUNTO(S)

cruzamento (genetica) genetica vegetal

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