Genetic variation

The species of the section Algarobia are the most intensively studied genetically among the sections of the genus Prosopis. Previous genetic studies on different species of the section Algarobia include studies of genetic variation and population structure (Solbrig and Bawa, 1975; Saidman, 1985, 1986; Saidman and Vilardi, 1987, 1993; Verga, 1995;

Cony, 1996; Saidman et al., 1997; Bessega et al., 2000b, c; Mantovan, 2004; Joseau, 2006), studies of interspecific geneflow (Saidman, 1990, 1993; Saidman et al., 1998a), and researches on the reproductive system of the species (Bessega et al., 2000a). These studies were conducted with morphological and physiological characters, isozymes markers, or also by means of different molecular markers such as RAPDs and RFLPs.

Different genetic studies performed in the species of section Algarobia showed that the species of this section exhibited a high genetic variability within populations and that they posses more genetic variation than species of section Strombocarpa (Pasiecznik et al., 2001). Besides, isoenzymatic and molecular studies conducted in different species of section Algarobia indicated a general trend towards significant homozygote excess within populations (Saidman 1986, 1988a, 1990, 1993; Saidman and Vilardi, 1987, 1993;

Saidman et al., 1997; Verga, 1995; Bessega et al., 2000 b). One of the most remarkable

characteristics of the section Algarobia is the high genetic similarity among the species so far studied (Saidman et al., 2000). The differentiation among species within this section, measured by means of biochemical or molecular markers, showed relatively low values, despite the important morphological differences that exist between them (Burkart, 1976). Almost all alleles of polymorphic isoenzyme loci are shared by all species. Diagnostic loci are mostly absent and species differ only in allele frequencies.

3.2.4. Hybridization

Another important aspect of the section Algarobia is that interspecific hybridization is very frequent in zones of sympatry. This hybridization creates intermediate phenotypes that render an accurate morphological determination difficult. Within the section Algarobia, a large number hybrids with of two or even three parental species have been postulated and confirmed with a variety of morphological, enzymic, and molecular studies in South America (Palacios and Bravo, 1981; Hunziker et al., 1986; Saidman, 1990; Verga, 1995; Vega and Hernández, 2005) and also in Mexico (Almanza et al., 1992). In Argentina, frequent interspecific hybridization has been confirmed between at least seven species of section Algarobia: P. alba, P. alpataco, P. caldenia, P. chilensis, P. flexuosa, P. nigra, and P. ruscifolia. Hybrid swarms composed by two or more of these species are described in zones of natural sympatry and also in areas where the natural habitat has been disturbed by human impact.

The strong evidences of natural hybridization among several species of the section Algarobia, as well as the high genetic similarity between them, has led to the assumption that these species would integrate a syngameon (Palacios and Bravo, 1981;

Saidman, 1985, 1988a, 1993; Saidman and Vilardi, 1987, 1993; Saidman et al., 1998a;

Bessega et al. 2000a, b). Grant (1981) defines the syngameon as “the most inclusive unit of interbreeding in a hybridizing species group”. Because frequent events of interspecific natural hybridization with fertile hybrid production in areas of sympatry occur, and isolation mechanisms between species seem to be weak or incomplete, the entities members of this syngameon can not be considered species under the classic

“biological species concept”, which defines species in terms of isolating mechanisms (Mayr, 1963; Dobzhansky, 1970; White, 1978). However, the members (botanical species) of the syngameon constitute real units in terms of morphology, ecology and evolution, and are considered “good taxonomic species”. Therefore, the entities

involved in this syngameon might be considered species under the Templeton (1989) cohesive concept that defines species as “the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability”.

Natural hybridization between Prosopis chilensis and P. flexuosa has been confirmed in sympatric areas of the Chaco phytogeographic province in Argentina (Verga, 1995).

Both species are highly related genetically, and significant gene flow occurs between them. Natural and fertile hybrids have also been described between these species as well as hybrids swarms in wide areas of sympatry. Morphological analysis performed by Verga (1995) in different populations located in contact areas between P. chilensis and P. flexuosa in the Chaco region revealed the natural occurrence of individuals with intermediated morphologies between both species. Besides, this morphological evidence for hybridization was confirmed by Verga (1995) by means of genetic studies with isozymes. Using the isozyme ADH (alcohol dehydrogenase), at locus ADH-A, which can be considered a diagnostic locus for species differentiation between P.

chilensis and P. flexuosa, Verga (1995) found that morphohybrids, i.e. individuals showing intermediate morphologies, presented also hybrid genotypes at the ADH-A locus. Hybridization rates appear to be higher in contact areas where the natural habitat has been disturbed by human impact. It is assumed an introgression process between these two species taking place in sympatric populations (Verga, 1995). In addition, controlled pollination experiments were successfully performed pollinating P. chilensis trees with pollen from P. flexuosa and the opposite (Córdoba Ana, pers. comm.).

It is unknown for how long the hybridization process between P. chilensis and P.

flexuosa has been taken place, but there is strong evidence that natural hybrids are widespread, fertile, and ancient. The spontaneous crossing between populations of both species is reflected in the Chaco phytogeographic province, Argentina, by the frequent occurrence of “hybrid swarms” in wide areas of sympatry (habitat overlap). These hybrid swarms are populations composed of pure individuals of both Prosopis species, and also by morphologically distinctive individuals that result from hybridization between the parent species (pure species), backcrossing of the offspring to members of the parent species, and interbreeding among the hybrid individuals, too. Hybrids swarms between P. chilensis and P. flexuosa are also very common in areas of sympatry

characterized by man-made disturbance, such as irrigation areas, or areas where natural forest was cut down to clear land for agriculture or ranching, and then abandoned.

In spite of hybridization, P. chilensis and P. flexuosa represent “real” biological units in terms of morphology, ecology, genetics and evolution, and both species have maintained and are maintaining genetic, phenotypic and ecological cohesion within them, and distinction between them, too. They have maintained themselves as distinct evolutionary lineages for million of years. However, the continuous production of hybrids might favor the occurrence of “evolutionary experiments” through the production of novel genetic combinations, and hybridization might have been important in the evolution of this group (Saidman et al., 1998b). Hybrids may display characteristics that allow them to occupy niches different from the parental ones.

Frequently, hybrids display special capabilities to exploit open or hybrid habitats (Anderson, 1949). Such specifically adapted populations could be used in forest genetic improvement programs to obtain basic material to restitute the plant cover in degraded areas.