Hybridization and genetic variation of Argentine species of Prosopis

Abstract The taxonomic division of the Prosopis genus, its geographic distribution, and the factors that limit it are briefly considered. All of these taxa except one are diploid (2 n = 28). Several species of sect. Algarobia are known to be self-incompatible and cross-fertilizing. The data on genetic variation concerning the percentage of polymorphic loci ( P ) and the expected mean of heterozygotes per locus ( H ) shows that arboreal species belonging to sect. Algarobia possess more genetic variation ( P = 38−50%, H = 0.132−0.223 ) than the shrubby species of sect. Strombocarpa ( P = 8.69−17.39%, H = 0.022−0.063 ). Within the first group, the ‘vinal’ ( P. ruscifolia ), an aggressive woody colonizer of northeastern Argentina and southern Paraguay, is the species that possesses the lowest measures of genetic variation ( P = 38%, H = 0.132 ± 0.044 ). It may be that the adaptive strategy of this noxious weed involves less genetic variability than that possessed by its congeners along with more phenotypic plasticity and wider ecological tolerance. The possibility that the species of Strombocarpa may be self-compatible is discussed; self-fertilization could explain the lower genetic variation displayed by their populations. Interspecific hybridization is frequent between species of Algarobia , and hybrids in general have regular meiotic pairing. No hybrids between Algarobia and Strombocarpa have been described so far. This and the strong differences in the mobility of certain enzymes between species belonging to one or the other section indicates that it would be more appropriate to raise the sections at least to the level of subgenera. Chromatography of phenolic compounds has been used with success by several authors for the characterization of species and identification of interspecific hybrids. A list of all natural Prosopis hybrids of section Algarobia so far cited is given (Table 1). The promising Argentine species for cultivation are P. alba, P. nigra, P. chilensis, P. flexuosa and P. hassleri and their most important uses are mentioned. Finally, the abusive exploitation of the ‘algarrobo’ woods in Argentina and the urgent need for genetic conservation of the most valuable and promising species is pointed out.