Beatriz O. Saidman

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.

Isoenzymes of Ganoderma species from southern South America

One hundred and thirty four dikaryotic isolates of Ganoderma, morphologically determined as G. adspersum, G. annulare, G. applanatum, G. brownii, G. lobatoideum, G. lobatum, G. lucidum, G. oerstedii, G. resinaceum, and G. tornatum, were examined by horizontal PAGE for eight enzymatic activities. A total of 94 bands were analysed. The objective was to evaluate the relatedness among the specific taxa by analysing numerically the isoenzymic patterns. To discern fungal individuals somatic incompatibility tests were used. Within each enzyme system several allozyme phenotypes could be observed. The enzymic systems analysed failed to yield diagnostic bands for each morphologically defined group, except the AKP system for G. resinaceum. Some bands were shared by all the isolates studied. On the other hand, diagnostic bands at the species complex level were encountered.

First comparative phenetic studies of Argentinean species of Acacia (Fabaceae), using morphometric, isozymal, and RAPD approaches

Morphological and genetic diversity among Acacia aroma, A. macracantha, A. caven, and A. furcatispina were studied with morphometric, isozymal, and RAPD approaches. The analysis of seven isozyme systems revealed 21 loci, and RAPD analysis showed 34 loci. Most of these loci allowed us to differentiate the species, with the exception of A. aroma and A. macracantha, the two most similar species. The levels of genetic variability estimated by isozymes were higher than those obtained from RAPD analyses. Morphometric characters showed highly significant differences among the species, although A. aroma and A. macracantha are differentiated only by thorn length. The phenogram obtained from isozyme data is consistent with morphological data. The RAPD phenogram based on allelic frequencies showed agreement with morphological and isozymal approaches only at the intraspecific levels, while the RAPD phenogram based on Nei and Lis similarity measures agreed with the phenograms constructed from isozyme and morphological data. High similarities and high indirect gene flow were found between A. aroma and A. macracantha, results that call the relationship between them into question.

Evolution of small inversions in chloroplast genome: a case study from a recurrent inversion in angiosperms

Small inversions (SIs) in the chloroplast genome of angiosperms are ubiquitous. These inversions are always flanked by inverted repeats (palindromes or quasipalindromes) between approximately 8 and 50 bp long that form a hairpin structure when the DNA is single‐stranded. We evaluated different methodological and empirical issues about SI evolution. As a case study, we analysed an SI recently discovered in the psbC–trnS intergenic region of Prosopis (Fabaceae). First, we analysed how inversions can be optimized in cases where the inverted segment also shows indels and substitutions, proposing a method based on Fixed States Optimization. Second, we evaluated the occurrence of this inversion on a phylogeny that includes the major lineages of angiosperms. Finally, we assessed whether the occurrence of this inversion was related to the thermodynamic stability of the hairpin structure (measured by its corresponding free energy) and/or the length of the palindromes by using a modified version of Maddison’s Concentrated Changes Test. Hairpin structure was conserved in most of the 154 sequences analysed, with the inversion taking place at least 10 times in different lineages (monocots, magnoliids, rosids). As was previously proposed for other SIs, our analysis strongly suggests that the occurrence of this inversion is correlated with higher hairpin stability. In contrast, we found no evidence of a correlation with longer palindromes. Our results are in agreement with the hypothesis that hairpin formation is a requisite for SI occurrence. However, alternative explanations cannot be discarded.

Mating system and population structure of Acacia aroma and A. macracantha (Fabaceae)

Acacia aroma and A. macracantha are closely related species that inhabit northern and central Argentina. The reproductive barriers between them seem to be weak. They exhibit low genetic differentiation, high levels of interspecific gene flow, and extensive areas of sympatry. Isoenzymatic approaches were used to evaluate the population structure and mating system parameters in natural Argentine populations of A. aroma and A. macracantha and to provide new tools for the analysis of relationships between these two species. All studied populations had high levels of genetic variability and no significant departures from Hardy-Weinberg expectations, but the two species did not differ from each other. Most variability occured within populations. Mating system analysis showed high levels of outcrossing, no biparental inbreeding, and a high probability that individuals within progeny arrays are full rather than half sibs. In all A. aroma and A. macracantha populations, polymorphic loci had the same allelic variants, and no geographic or genetic isolation between species was found. The results favor the hypothesis that these two entities represent a single polymorphic species rather than two distinct species.

Consistency between marker- and genealogy-based heritability estimates in an experimental stand of Prosopis alba (Leguminosae)

Prosopis represents a valuable forest resource in arid and semiarid regions. Management of promising species requires information about genetic parameters, mainly the heritability (h(2)) of quantitative profitable traits. This parameter is traditionally estimated from progeny tests or half-sib analysis conducted in experimental stands. Such an approach estimates h(2) from the ratio of between-family/total phenotypic variance. These analyses are difficult to apply to natural populations of species with a long life cycle, overlapping generations, and a mixed mating system, without genealogical information. A promising alternative is the use of molecular marker information to infer relatedness between individuals and to estimate h(2) from the regression of phenotypic similarity on inferred relatedness. In the current study we compared h(2) of 13 quantitative traits estimated by these two methods in an experimental stand of P. alba, where genealogical information was available. We inferred pairwise relatedness by Ritlands method using six microsatellite loci. Relatedness and heritability estimates from molecular information were highly correlated to the values obtained from genealogical data. Although Ritlands method yields lower h(2) estimates and tends to overestimate genetic correlations between traits, this approach is useful to predict the expected relative gain of different quantitative traits under selection without genealogical information.

Genetic divergence among species of the section Strombocarpa, genusProsopis (Leguminosae)

We studied degree of variability and relationships among populations belonging to five species of the section Strombocarpa of the family Leguminosae, namelyProsopis ferox, P. torquata, P. pubescens, P. strombulifera andP. reptans, by isozyme electrophoresis. The genetic similarity betweenP. reptans andP. strombulifera is high enough for the two to be considered subspecies or geographic races ofP. strombulifera instead of good taxonomic species. As expected these species cluster withP. pubescens, all belonging to series Strombocarpae. This group is different isoenzymatically from the remaining species,P. torquata andP. ferox. Comparison of our results with those obtained earlier for species of the section Algarobia showed that genetic variability in the section Strombocarpa is lower than in the section Algarobia, while the species divergence is much higher. These results suggest different adaptive strategies between these sections.

Argentinean native wood species: Physical and mechanical characterization of some Prosopis species and Acacia aroma (Leguminosae; Mimosoideae).

One of the problems in marketing the wood of Prosopis and Acacia is the lack of standardization of its qualities. The aim of this paper was to obtain a preliminary detection of some properties of the wood of four species of the genus Prosopis and one species from Acacia grown in Argentina. To accomplish this objective, the content of extractives and some physical and mechanical characteristics were analyzed. The density rho(12) of all the species indicates that these woods range from heavy to very heavy (>or=0.69g/cm(3)). The total volumetric shrinkage values are low, less than 10%, for all species. The parallel compression strength and the shear strength for all the species indicate a very resistant wood (>or=46.93MPa and >or=18.35MPa, respectively). Brinell hardness was higher than 5kg/mm(2) in all cases. The species with less content of extractives is P. ruscifolia (approximately 9% of anhydrous mass) whereas A. aroma was the one with the greatest content (approximately 25% of anhydrous mass in the heartwood).

First report on RAPDs patterns able to differentiate some Argentinean species of section Algarobia (Prosopis, Leguminosae)

Prosopis species constitute a very important resource in arid and semiarid regions. Some species of section Algarobia hybridise and introgress naturally in areas of sympatry. According to previous isoenzymatic studies these species have high variability within populations. However, the genetic differentiation among species was very low, and these markers failed to provide diagnostic loci for species recognition. Here we analysed by the RAPD technique natural populations of Prosopis alba, P. ruscifolia, P. nigra, P. flexuosa, and P. vinalillo with the purpose of obtaining markers for species and hybrid identification, by analysing the distribution of genetic diversity within and among species. Genetic variability (H= 0.12–0.26) was similar in all populations. Genetic differentiation among populations (FST= 0.39) was highly significant. Hierarchical analysis of genetic structure performed by Wright (1978) method and analysis of molecular variance (AMOVA) indicated that the diversity among populations within species is low (4–13%) and most genetic diversity (54–61%) occurs within populations. The differentiation among species is intermediate (26–42%) between the previous components but highly significant. Five bands provided a tool for identifying any of the species studied, with the exception of P. vinalillo. The difficulty in diagnosing this species is discussed in reference to the hypothesis of its hybrid origin.

New microsatellite loci for Prosopis alba and P. chilensis (Fabaceae)

Premise of the study: As only six useful microsatellite loci that exhibit broad cross-amplification are so far available for Prosopis species, it is necessary to develop a larger number of codominant markers for population genetic studies. Simple sequence repeat (SSR) markers obtained for Prosopis species from a 454 pyrosequencing run were optimized and characterized for studies in P. alba and P. chilensis. Methods and Results: Twelve markers that were successfully amplified showed polymorphism in P. alba and P. chilensis. The number of alleles per locus ranged between two and seven and heterozygosity estimates ranged from 0.2 to 0.8. Most of these loci cross-amplify in P. ruscifolia, P. flexuosa, P. kuntzei, P. glandulosa, and P. pallida. Conclusions: These loci will enable genetic diversity studies of P. alba and P. chilensis and contribute to fine-scale population structure, indirect estimation of relatedness among individuals, and marker-assisted selection.