ALLOZYME POLYMORPHISMS, OUTCROSSING RATES, AND HYBRIDIZATION OF SOUTH-AMERICAN NOTHOFAGUS

Electrophoretically detectable genetic variation was used to describe the genetic structure of three South American species of Nothofagus: t he widespread N. betuloides and N. dombeyi, and the geographically res tricted N. nitida. Although the widespread species possess higher leve ls of genetic variation, the three species have more genetic variation within than among populations. These results are consistent with the theoretical expectations for woody, presumably highly outcrossed speci es with wind-borne seeds. Estimates of outcrossing rates from progeny arrays yielded slightly higher average t-values for N. nitida (1.158) and N. dombeyi (range 0.873-1.045) than for N. betuloides (0.878). Hie rarchical analysis of population structure revealed values of F-IS and F-IT that were positive and significantly different from zero at most loci and for each species. The levels of inbreeding detected by F-sta tistics indicate some degree of self-fertilization and/or population s ubstructuring into discrete family groups. Reduced seed vagility and r egeneration of natural stands after disturbance by a few remnant indiv iduals would probably generate the recruitment of related seedlings un derneath parent trees. The analysis of a putative hybrid population be tween N. nitida and N. betuloides indicated that individuals clearly s egregated into nitida-like or betuloides- like individuals. The reduce d outcrossing rate of betuloides-like individuals from the hybrid site (t = 0.585) is interpreted in concert with low pollen availability an d the increased probability of selfing and/or hybridization in Nothofa gus.