Isozyme Variation in a Tropical Pioneer Tree Species (Cecropia obtusifolia, Moraceae) with High Contents of Secondary Compounds1

Abstract

ABSTRACTWe define 48 allozyme loci for a tropical pioneer tree species, Cecropia obtusifolia Bertol, which has high contents of secondary compounds. Our goals were to find the effects of extraction procedures on artifacts and variation in resolution of enzyme banding patterns; to explore the relationship among the variation of the loci sampled and the enzymes' molecular structure, metabolic function and substrate; to obtain estimates of the genetic variation in this species at Los Tuxtlas rain forest (México) and to explore the variation of allelic frequencies in six successive life‐history stages of the species.The resolution of the isozymes bands and the actual banding pattern varied with the type and age of tissue, the collection and storage procedures, the extraction buffer, and other loading and running procedural details. However, artifactual variation was eliminated with a new extraction buffer for species with high contents of secondary compounds.Of the 26 enzymes resolved for C. obtusifolia, we found that enzymes with a greater number of substrates and an oligomeric quaternary structure tended to be more variable than their counterparts, but the relationship was not statistically significant. The proportion of polymorphic systems varied significantly with the metabolic pathway and the function of the enzymes. Enzymes involved in starch synthesis are significantly more variable (p < 0.05) than all others, except those involved in amino acids metabolism and the proportion of polymorphic enzymes is also significantly associated with the hnction of the enzyme, the hydrolases and isomerases are significantly more variable than lyases and oxidoreductases enzymes.The percentage of polymorphic loci for C. obtusifolia was estimated at P = 27.1%. The effective number of alleles was estimated at ne = 1.3 and ne = 2.4 for all loci and only polymorphic ones respectively and the average heterozygosity (H) for all 48 loci was estimated at H = 0.05. Allele frequencies varied throughout the life‐cycle of the species, with significant differences for some alleles and loci among some life‐cycle stages. “Tree seeds” allele frequencies differ significantly (P < 0.05) from “rainy dispersed seeds” in 7 of 8 loci and from “soil seeds” in Six of eight loci. Allele frequencies of all three seed categories (“tree seeds”, “rainy dispersed seeds”, and “soil seeds”) differed strongly from established individuals (seedlings, juveniles and adults), while allele frequencies of established individuals are relatively similar to one another. Seedling allele frequencies at most loci were also significantly different from those found in seeds collected from trees, seed‐rain, and soil. Two alleles (at GOT‐2 and FE‐2) were only found in soil seeds and one allele (at LAP‐2) was only found in seedlings.