Pollen contamination and mating patterns in a prosopis alba clonal orchard: Impact on seed orchards establishment

Abstract

Prosopis alba (Leguminosae) is an important species from ecologic and economical points of view in arid and semi-arid regions of Argentina. In several open-pollinated species, pollen contamination from off-orchard parents and selfing have been proven to reduce orchard seed quality. In 2002, the first clonal orchard of Prosopis alba was established in Fernández (Santiago del Estero, Argentina) with 12 trees phenotypically selected from a progeny trial, based on height, pod production per year and pod sweetness. The aim of this study was to evaluate the mating patterns and pollen contamination rate in the orchard using ten SSR markers and paternity analysis. All the clones together with the progeny of a single clone (open-pollinated seeds) were genotyped. Data was processed by two different methods based on likelihood and Bayesian approaches, respectively. A high consistency (89%) of results was observed between the two methods, and pollen contamination rate was estimated between 27% and 37%. The minimum number of different pollen donors per mother plant varied from three to five and selfing occurrence was low (<1.6%). Based on the estimated status number (Ns = 4.4), the expected coancestry in the seed crop is equal to a Mendelian population with an effective size of 4-5 individuals. Genetic analyses are encouraged during the establishment and monitoring of trials in forest breeding and management programmes. It is strongly recommended to establish seed orchards in isolated areas and to guarantee equal representation of parental genotypes in the orchards.