The section Algarobia of genus Prosopis involves important natural resources in arid and semiarid regions of the world. Their rationale use requires a better knowledge of their biology, genetics and mating system. There are contradictory information about their mating system. Some authors claim they are protogynous and obligate outcrosser. However, some evidence have been shown indicating that they might not be protogynous and that they might be somewhat self-fertile. The current paper analyses genetic structure and mating system parameters in populations of seven species of this section from South and North America based on isozyme data. In all species a significant homozygote excess was found in the offspring population but not in mother plant genotypes. Multilocus and mean single locus outcrossing rates (tm, ts) indicated that about 15% selfing can occur in the studied populations. The heterogeneity between pollen and ovule allele frequencies was low suggesting population structuration, in agreement with the estimates of correlation of tm within progeny (rt) and correlation of outcrossed paternity (rp). The difference of FIS estimates between offspring and mother plants suggest some selection favouring heterozygotes between seedling and adult stages.

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The objective of this study consisted in the characterization of morphological variability presented in the Prosopis population in the corridor that unites the semiarid with the arid region in Argentina. In this region, four species of Prosopis get in contact (P. chilensis, P. flexuosa, P. alba and P. nigra). Due to the fact that these species make fertile hybrids, this corridor has an enormous diversity. The mother's allowed the differentiation of five morphologic groups, which from the point of view of classic taxonomy have been classified in 16 taxa. Only groups 3 and 4 presented intermediate characteristics. The morphologic study of the descendants showed that the regrouping of the intermediate groups had a high error rate. This fact reaffirms the condition of intermediate groups. The canonical correlation analysis between the morphologic variables of the mothers and the seedlings revealed that each morphologic group had its own association of canonical variables between mothers and their seedlings, and that the variability observed in the set of the mothers and their seedlings follows a certain order. The enormous existing morphologic diversity in the Prosopis populations of the semi-arid Chaco of the north of Córdoba and southeast of Santiago del Estero was ordered in five morphologic groups. The descendant's morphologic studies confirmed the existence of the defined morphologic-genetic groups.

Genetic variation was investigated in ¢ve subtidal locations from the north of San Mat|¤ as Gulf (42823 0 S) to the north of San Jorge Gulf (45802 0 S). Fifteen loci were studied by means of vertical starch gel electrophoresis; ten loci were polymorphic. The percentages of polymorphic loci (P 0.99 and P 0.95 criteria) varied between 40% and 53.33% and between 33.33% and 46.67%, respectively. Expected mean heterozygosity per locus (He) ranged from 0.128 (Bajo Oliveira-El So¤ tano) to 0.160 (Puerto Madryn); this range is similar to those observed for other scallop species. All the loci were in Hardy^Weinberg equilibrium. The estimated genetic distances among populations were signi¢cantly lower than those reported for comparisons between populations of di¡erent pectinid species, suggesting that the ¢ve populations studied belong to the same species (Aequipecten tehuelchus). The absence of exclusive ¢xed alleles also supported that assumption. The results obtained suggest: (i) assignment of species or subspecies status to some morphological variants of the shells, like the 'madrynensis' form is not supported; (ii) the commercially exploited Tehuelche scallop from San Mat|¤ as and San Jose¤ Gulfs should be managed as distinct stocks.

Worldwide, large areas of forest are being transformed to other land cover types and the resulting fragmented populations may suffer from restricted gene flow leading to genetic pauperization and increased inbreeding. To assess the genetic constitution of fragmented Polylepis australis mountain forests of central Argentina, analyses of the structure and diversity of ISSR markers were carried out for 90 trees distributed throughout five river basins with differing degrees of fragmentation. Overall, average polymorphism (P) ranged between 87.2 and 94.9% (95% criterion) while marker diversity index (M) varied between 0.35 and 0.39; values which are comparable with other wind-pollinated tree species. Analysis of molecular variance (AMOVA) revealed that most genetic variation occurred within river basins (97.8%), with only a little occurring between river basins (2.2%; ΦST = 0.02). In addition, Mantel’s test indicated that P. australis does not follow the usual pattern of isolation by distance; instead the UPGMA method showed that trees from the two most degraded river basins formed a group while trees from the three better preserved basins formed another. As such, either effective pollen flow has maintained high levels of genetic diversity, or present day genetic variability is a remnant of a recently fragmented ancestral panmictic population. We conclude that, at present, genetic degradation in P. australis populations of central Argentina is not as important as ecological degradation – such as soil loss, intensive browsing by livestock or increased frequencies of wildfires, and that genetic variability is still fully available for forest restoration.

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The ''algarrobo'' [Prosopis chilensis (Molina) Stuntz] is a tree species that represents an important natural resource in arid and semi-arid regions of Argentina. In this paper, we analysed and compared the variability of 46 RAPD (Random Amplified Polymorphic DNA) loci with previous estimates obtained from 12 isozyme markers in nine Argentinean populations of P. chilensis representative of the whole range of this species in Argentina. We evaluated the population structure and the existence of genetic variants associated with environmental variables. Expected heterozygosity (H e ) estimated from RAPD varied significantly among populations and regions. Hierarchical analysis of genetic variability (AMOVA) showed that most (88.1%) of the total diversity occurs within populations, the component among populations within regions (9.3%) was intermediate, while the between-region component was the lowest (2.6%). All three variance components were highly significant. The MDS plot from pair-wise U ST matrix was consistent with the highly significant among-region differentiation indicated by the AMOVA. All 12 variable isozyme loci and 26 out of 46 RAPD loci showed significant or highly significant association with at least one geographic/climatic variable according to the stepwise multiple regression analysis. These results imply that the genetic differentiation among populations is better explained by environmental or biogeographical grounds than by geographical distances, suggesting gametic disequilibrium with loci responsible for the adaptation to particular environmental conditions. The information from RAPD markers would provide a relevant criterion to preserve genetic diversity in programmes for conservation and rationale use of this species. Communicated by R. Matyssek. Electronic supplementary materialThe online version of this article (