247
species (Verga et al. 2014) and high morphological similarities among certain species (Burkart 1976). Moreover, interspecific hybridization creates intermediate phenotypes (Burkart 1976; Palacios and Bravo 1981; Hunziker 1986; Saidman 1990;
Verga 1995; Teich et al. 2015), hampering their taxonomical classification. It has
even been proposed that the complex of Prosopis species in the Great American
Chaco may conform a single evolutionary unit (syngameon) (Verga et al. 2014) in
which their ability to exchange genetic information may act as a mechanism that
increases their variability and evolutionary potential, generating high diversity in
contact areas. In this context, taxonomic methods based on the subjective observation of morphological traits, such as systematic keys (Burkart 1976), may not be
enough to classify individuals. On the other hand, numerical taxonomy, based on
leaf and fruit traits, has proved to be very useful to obtain groups of Prosopis individuals in a much greater degree of detail than that achieved through classical systematics, even among species with similar genetic characteristics (Verga 1995;
Joseau and Verga 2005; Verga and Gregorius 2007).
Traditionally, Prosopis species in the Chaco region are classified in two large
groups: the White algarrobos or “algarrobos blancos” (WA) and the Black algarrobos or “algarrobos negros” (BA). Though this classification is not taxonomical,
these two groups are easily differentiated by the characteristics of the fruits, being
the two main differences that WA fruits are yellowish and thinner, with less mesocarp, than BA fruits, which are always dark. At a macrogeographic scale, species
within each group are not sympatric; they replace each other establishing contact
areas. On the contrary, black and white algarrobos coexist, although at the microgeographic scale, they occupy different niches (Vega et al. 2020).
Figure 9.1 shows the central areas of distribution of the pure species of white
algarrobos in Argentina. As a whole, they occupy the totality of the Chaco region,
always depending on the local environmental conditions that determine their presence or absence on a more detailed scale. The distribution of P. chilensis appears
fragmented because its distribution depends to a great extent on the existence of
water in the subsoil, so this species in the Chaco is always associated with the foothills of the Sierras Pampeanas. In the central part of the arid Chaco, P. flexuosa
(black algarrobo) and Aspidosperma quebracho-blanco (the white quebracho) are
dominant, while contact areas between P. flexuosa and P. chilensis appear on the
piedmont. In the rest of the Chaco, the white algarrobos are in contact with P. nigra.
To the north of the Chaco, P. ruscifolia (“vinal”) appears completing this complex
of species (Verga et al. 2009).
Many genetic, ecological, and evolutionary studies of Prosopis species in the
Chaco have used a set of leaf and fruit traits to characterize and identify groups
(Burkart 1976; Pasiecznik et al. 2001; Bessega et al. 2009). Regarding leaf traits, the
most frequently used are petiole length (PEL), number of pairs of pinnae (NPI),
pinna length (PIL), number of pairs of leaflets per pinna (NLP), leaflet length (LEL),
leaflet width (LEW), leaflet area (LEA), leaflet apex (LAPX), leaflet length/leaflet
width (LEL/LEW), and leaflet apex/total area (LAPX/LEA) (Fig. 9.2). Fruit morphometric characters usually include fruit length (FrL), fruit width (FrW), and fruit
thickness (FrTh) (Fig. 9.2). Prosopis fruits are also characterized according to their
9 Genetic Variation Patterns of “Algarrobos” from the “Great American Chaco…
species (Verga et al. 2014) and high morphological similarities among certain species (Burkart 1976). Moreover, interspecific hybridization creates intermediate phenotypes (Burkart 1976; Palacios and Bravo 1981; Hunziker 1986; Saidman 1990;
Verga 1995; Teich et al. 2015), hampering their taxonomical classification. It has
even been proposed that the complex of Prosopis species in the Great American
Chaco may conform a single evolutionary unit (syngameon) (Verga et al. 2014) in
which their ability to exchange genetic information may act as a mechanism that
increases their variability and evolutionary potential, generating high diversity in
contact areas. In this context, taxonomic methods based on the subjective observation of morphological traits, such as systematic keys (Burkart 1976), may not be
enough to classify individuals. On the other hand, numerical taxonomy, based on
leaf and fruit traits, has proved to be very useful to obtain groups of Prosopis individuals in a much greater degree of detail than that achieved through classical systematics, even among species with similar genetic characteristics (Verga 1995;
Joseau and Verga 2005; Verga and Gregorius 2007).
Traditionally, Prosopis species in the Chaco region are classified in two large
groups: the White algarrobos or “algarrobos blancos” (WA) and the Black algarrobos or “algarrobos negros” (BA). Though this classification is not taxonomical,
these two groups are easily differentiated by the characteristics of the fruits, being
the two main differences that WA fruits are yellowish and thinner, with less mesocarp, than BA fruits, which are always dark. At a macrogeographic scale, species
within each group are not sympatric; they replace each other establishing contact
areas. On the contrary, black and white algarrobos coexist, although at the microgeographic scale, they occupy different niches (Vega et al. 2020).
Figure 9.1 shows the central areas of distribution of the pure species of white
algarrobos in Argentina. As a whole, they occupy the totality of the Chaco region,
always depending on the local environmental conditions that determine their presence or absence on a more detailed scale. The distribution of P. chilensis appears
fragmented because its distribution depends to a great extent on the existence of
water in the subsoil, so this species in the Chaco is always associated with the foothills of the Sierras Pampeanas. In the central part of the arid Chaco, P. flexuosa
(black algarrobo) and Aspidosperma quebracho-blanco (the white quebracho) are
dominant, while contact areas between P. flexuosa and P. chilensis appear on the
piedmont. In the rest of the Chaco, the white algarrobos are in contact with P. nigra.
To the north of the Chaco, P. ruscifolia (“vinal”) appears completing this complex
of species (Verga et al. 2009).
Many genetic, ecological, and evolutionary studies of Prosopis species in the
Chaco have used a set of leaf and fruit traits to characterize and identify groups
(Burkart 1976; Pasiecznik et al. 2001; Bessega et al. 2009). Regarding leaf traits, the
most frequently used are petiole length (PEL), number of pairs of pinnae (NPI),
pinna length (PIL), number of pairs of leaflets per pinna (NLP), leaflet length (LEL),
leaflet width (LEW), leaflet area (LEA), leaflet apex (LAPX), leaflet length/leaflet
width (LEL/LEW), and leaflet apex/total area (LAPX/LEA) (Fig. 9.2). Fruit morphometric characters usually include fruit length (FrL), fruit width (FrW), and fruit
thickness (FrTh) (Fig. 9.2). Prosopis fruits are also characterized according to their
9 Genetic Variation Patterns of “Algarrobos” from the “Great American Chaco…