Isozyme Polymorphism in the Leguminous Genus Leucaena

Abstract

The leguminous genus Leucaena includes 15 species, several of which have become pantropical owing to their importance as fodder, fuelwood, shade and ornamental trees. In all present study, protocols for obtaining reproducible isozyme phenotypes in Leucaena were developed for six isozyme systems (AGO, ADH, IDH, MDH, PGI and PGI) using Lhistidine and citric acid for running and tray buffers with cotyledon tissues. Standard descriptions of isozymic phenotypes were established using accession K997 of a local Hawaiian population L. leucocephala ssp. leucocephala as a control. Ontogenetic studies revealed that ADH phenotypes were developmentally variable. However, the phenotypes of other isozyme systems were not variable ontogenetically. One hundred sixty-nine accessions of 12 Leucaena species, including 3 tetraploid species and 9 diploid species, were analyzed for six isozyme systems. Extensive polymorphism was observed among the species, and the weighted polymorphism (Pw) was estimated. The higher Pw values characterized outcrossing species and the lower PwS were among self-pollinated species. Five of these isozyme systems (AGO, ADH, IDH, MDH and PGI) provided bands that were found useful in identifying taxa within the genus. These systems were also effective in detecting inter- and intra-specific hybrids. Three loci of AGO and two loci of IDH in the sibcrosses of L. lanceolata (K10) were confirmed. Seventy-nine accessions of "common" L. leucocephala ssp. leucocephala from tropical countries and thirty-six of "giant" L. leucocephala ssp. glabrata, mainly from Central America were tested for isozyme variation. The uniformity of isozymic expression of all "common" types suggests that they are offsprings of a "pure" line variety or a single tree. In contrast, the "giant" types of L. leucocephala ssp. glabrata were quite diverse. The "giant" type trees could be easily distinguished from the "common" in the AGO system. The ancestral relationship of Leucaena diploids and polyploids were analyzed using the electrophoresis technique. The results did not support the hypotheses that (1) L. diversifolia ssp. diversifolia (2n=104) is an autotetraploid from L. diversifolia ssp. trichandra (2n=52) and (2) L. pallida (2n=104) is an amphidiploid from L. diversifolia ssp. trichandra and L. esculenta (2n=52).