Mating system parameters and population genetic structure in pioneer populations of Prosopis velutina (Leguminosae)

This study describes the genetic structure and provides estimates of mating system parameters in three natural populations of Prosopis velutina Woot. in southeastern Arizona. F statistics derived from isozyme data revealed the presence of both interpopulation and intrapopulation genetic differentiation. This population structure is discussed in relation to the ecological history of these populations that invaded the grasslands sites from adjacent riparian areas within the last 90 years, and possible modes of seed dispersal. The multilocus estimation program MLT of Ritland ( Journal of Heredity 8: 235–237, 1990) was used to provide estimates of t s , t m , and the fixation index ( F ) of the maternal parents. Average estimates of t s ranged from 0.591 to 0.912. Estimates of t m ranged from 0.609 to 1.004, and averaged 0.758. The difference between t m and t s , which provides a measure of biparental inbreeding, averaged 0.058. This last result, together with significant heterogeneity found in pollen allele frequencies, suggests that a family structure has developed in the populations. A negative F value for one of the populations suggests selection against homozygotes between the seedling stage and maturity.