253
9.3.1 Morphological and Molecular Characterization of a
Hybrid Zone between Prosopis alba and P. nigra in the
Chaco Region of Northwestern Argentina
Due to its wide distribution, P. alba displays multiple zones of contact with different
congeners throughout its geographic range. One of these congeneric species is
P. nigra, which has good capacity to settle in highly degraded sites (Demaio et al.
2002), supporting drier soils than P. alba.
The study was carried out near Santa Victoria town, in northwestern Argentina
(22° 16′ S; 62 42′ W). Three discontinuous sites were sampled: sites 1 and 2, where
pure morphotypes of P. alba and P. nigra, were present, respectively, and site 3, a
zone where both intermediate and pure morphotypes were found (i.e., a putative
hybrid swarm). The three studied sites constitute patches of forests without continuous forested masses connecting them. This co-distribution area provides a suitable
natural experimental design for understanding hybridization between P. alba and
P. nigra and its consequences at the phenotypic level. In addition, the larger sample
size included in our study (138 from the hybrid site) with respect to the previous one
(it included just 7 hybrids, Vega and Hernandez 2005) allowed us to evaluate the
spatial structure and the association between genetic and morphological variation,
which has been very useful in the study of other hybrid swarms of Prosopis (Teich
et al. 2015).
Through the morphological classification of leaves and fruits it was possible to
differentiate the individuals belonging to the pure species of P. alba and P. nigra and
to morphological groups formed by individuals with intermediate morphology to
both species. In the individuals of the pure sites of P. alba and P. nigra, an evident
morphological identity was found, where the leaf type characteristic of P. alba corresponds to the characteristic fruit of this species, occurring the same in P. nigra.
However, in site 3 belonging to the hybrid swarm, it was observed that there is an
uncoupling in the type of leaf and fruit, that is, individuals with leaves of P. nigra
and fruit of P. alba appeared. According to the literature, this is the first time that
this type of decoupling has been detected in fruit and leaf characters. In other
hybridization studies carried out in P. chilensis and P. flexuosa (Verga 1995; Joseau
2006), the hybrids presented intermediate characteristics to both pure species, that
is, the decoupling between fruit and leaf variables detected here was not observed.
In addition, a genetic characterization was performed through 6 microsatellite
loci. Genetic differentiation among sites was significant (P < 0.001), with the greatest observed divergence between pure sites, site 1 and 2 (F ST = 0.305), whereas the
hybrid site showed an intermediate differentiation (F ST = 0.087 and 0.092 between
sites 1 and 3, and between sites 2 and 3, respectively).
It has been shown in different plant species that molecular and morphological
assignments do not always correlate and that phenotypically intermediate individuals are the result of a wide intraspecific variation or convergent evolution (Rieseberg
et al. 1993; Craft et al. 2002; Curtu et al. 2007; Moran et al. 2012) rather than of
9 Genetic Variation Patterns of “Algarrobos” from the “Great American Chaco…
9.3.1 Morphological and Molecular Characterization of a
Hybrid Zone between Prosopis alba and P. nigra in the
Chaco Region of Northwestern Argentina
Due to its wide distribution, P. alba displays multiple zones of contact with different
congeners throughout its geographic range. One of these congeneric species is
P. nigra, which has good capacity to settle in highly degraded sites (Demaio et al.
2002), supporting drier soils than P. alba.
The study was carried out near Santa Victoria town, in northwestern Argentina
(22° 16′ S; 62 42′ W). Three discontinuous sites were sampled: sites 1 and 2, where
pure morphotypes of P. alba and P. nigra, were present, respectively, and site 3, a
zone where both intermediate and pure morphotypes were found (i.e., a putative
hybrid swarm). The three studied sites constitute patches of forests without continuous forested masses connecting them. This co-distribution area provides a suitable
natural experimental design for understanding hybridization between P. alba and
P. nigra and its consequences at the phenotypic level. In addition, the larger sample
size included in our study (138 from the hybrid site) with respect to the previous one
(it included just 7 hybrids, Vega and Hernandez 2005) allowed us to evaluate the
spatial structure and the association between genetic and morphological variation,
which has been very useful in the study of other hybrid swarms of Prosopis (Teich
et al. 2015).
Through the morphological classification of leaves and fruits it was possible to
differentiate the individuals belonging to the pure species of P. alba and P. nigra and
to morphological groups formed by individuals with intermediate morphology to
both species. In the individuals of the pure sites of P. alba and P. nigra, an evident
morphological identity was found, where the leaf type characteristic of P. alba corresponds to the characteristic fruit of this species, occurring the same in P. nigra.
However, in site 3 belonging to the hybrid swarm, it was observed that there is an
uncoupling in the type of leaf and fruit, that is, individuals with leaves of P. nigra
and fruit of P. alba appeared. According to the literature, this is the first time that
this type of decoupling has been detected in fruit and leaf characters. In other
hybridization studies carried out in P. chilensis and P. flexuosa (Verga 1995; Joseau
2006), the hybrids presented intermediate characteristics to both pure species, that
is, the decoupling between fruit and leaf variables detected here was not observed.
In addition, a genetic characterization was performed through 6 microsatellite
loci. Genetic differentiation among sites was significant (P < 0.001), with the greatest observed divergence between pure sites, site 1 and 2 (F ST = 0.305), whereas the
hybrid site showed an intermediate differentiation (F ST = 0.087 and 0.092 between
sites 1 and 3, and between sites 2 and 3, respectively).
It has been shown in different plant species that molecular and morphological
assignments do not always correlate and that phenotypically intermediate individuals are the result of a wide intraspecific variation or convergent evolution (Rieseberg
et al. 1993; Craft et al. 2002; Curtu et al. 2007; Moran et al. 2012) rather than of
9 Genetic Variation Patterns of “Algarrobos” from the “Great American Chaco…