Cytogenetic studies in Phaseolus L . ( Fabaceae )

A review of the cytogenetic studies carried out on Phaseolus as well as the different proposals that have been suggested to explain the chromosomal changes in the group are presented. The importance of including wild species in cytogenetic studies and the collaboration between taxonomists and cytogeneticists in order to draw better conclusions are emphasized.


INTRODUCTION
The family Fabaceae (Leguminosae) contains the sub-family Papilionoideae of which the tribe Phaseoleae is one of the most important groups because it contains genera such as Glycine (soybean), Phaseolus (American beans) and Vigna (Asiatic beans), which are economically important due to their role in human nutrition and their use as cattle forage and ornamental (Lackey, 1981).
In his original description of Phaseolus, Linnaeus (1753) included eleven species, but with time the number grew to 200, distributed both in the Old and the New World.In 1970, Verdcourt redefined Phaseolus, considering it exclusively of New World origins, with approximately 50 species whose characteristics are similar to those of P. vulgaris (generitype).This redefinition was confirmed as valid and refined by a series of studies by other researchers (Maréchal et al., 1978and Lackey, 1981, 1983).
The last revision of the genus was made by Delgado-Salinas (1985), who recognized only 36 species in North and Central America.Despite the taxonomic studies carried out on the genus that have led to its clear delimitation, neither the number of taxa of which the genus is composed nor the genetic relationship between species has been well established (Debouck, 1991).Delgado-Salinas (1985) estimates that the genus contains 36 species, in North and Central America, some of them with subspecific divisions, while Debouck (1991) includes 52 especies, without subspecific divisions.
Although the importance of cytogenetic studies have been noted by several authors (Thomas, 1973;Green et al., 1980;Almeda and Chuang, 1992), most studies have dealt with economically important species, ignoring the potential of wild species and relating only to cultivated species such as Phaseolus.

CHROMOSOMAL STUDIES
The first reports on chromosome numbers in Phaseolus go back to 1925, when Karpetschenko obtained 2n = 22 for P. acutifolius A. Gray, P. coccineus L., P. lunatus L. and P. vulgaris L. From then on, a large number of cytogenetic studies have focused mainly on the determination of chromosome numbers, establishing x = 11 as the basic number.
Prior to 1996, of the approximately 50 species recognized in the genus Phaseolus, only 9 species and 4 subspecies had been chromosomally counted.Mercado-Ruaro andDelgado Salinas (1996, 1998) increased the number of taxa analyzed to 31.Based on the published literature Lackey (1979), Goldblatt (1981) and Mercado-Ruaro andDelgado Salinas (1996, 1998) propose that, as in the tribe Phaseoleae, the basic chromosome number in the genus is x = 11, with a haploid number of n = 10 in three species (P.leptostachyus Benth., P. micranthus Hook.& Arn., and P. macvaughii A. Delgado, ined. (Mercado-Ruaro and Delgado-Salinas, 1998)).The number of species that have been analyzed is very low, and the analyses have been restricted mainly to cultivated species (Sarbhoy, 1977;Joseph and Bouwkamp, 1978;Sinha and Roy, 1979a;Zheng et al., 1991).Mercado-Ruaro and Delgado-Salinas (1998) reported the karyotypic analysis of 10 wild species, that represent on average 20% of those comprising the genus.The lack of karyologic studies in the genus has been attributed to the reduced size of the chromosomes, which makes the analysis difficult (Hucl and Scoles, 1985;Zheng et al., 1991).Nonetheless, the available information has shown that there is a predominance of metacentric and submetacentric chromosomes, which translates into very symmetrical karyotypes.

Abstract
A review of the cytogenetic studies carried out on Phaseolus as well as the different proposals that have been suggested to explain the chromosomal changes in the group are presented.The importance of including wild species in cytogenetic studies and the collaboration between taxonomists and cytogeneticists in order to draw better conclusions are emphasized.
of Phaseolus has evolved towards an asymmetry, with a decrease in the total chromatin content.Mercado-Ruaro and Delgado-Salinas (1998), after encountering three aneuploid species with 2n = 20, have pointed out that aneuploidy has also played a role in the evolution of the karyotype.

GENOMIC HYBRIDIZATION
Studies of Phaseolus vulgaris by Frediani et al. (1993) using in situ hybridization have shown the position of the genes that code for polygalacturonase-inhibiting protein (PGIP) and stablished that the coding sequences are located in the heterochromatic pericentromeric region of metacentric chromosome 10, while Schumann et al. (1990) and Nenno et al. (1993) have documented the position of the phaseolin gene.In P. coccineus Avanzi et al. (1972) have located the ribosomal cistrons in the nucleolar and satellite regions of chromosomal pairs I and V using tritiumlabelled rRNA.These studies, all employing polytene chromosomes, show the potential of in situ hybridization for chromosome mapping.
The application of genomic in situ hybridization to taxonomy and the elucidation of genetic relationships are exemplified by the studies of Mercado-Ruaro on the Phaseolus vulgaris-P.coccineus complex, which investigated the possible hybrid origin of P. coccineus subsp.darwinianus Hernández X. & Miranda C. (= P. polyanthus Greenm.) as well as the genetic relationships between the species and subspecies that make up the complex.The results of this study have shown the high degree of genetic homology between the members of this group, and because of this it was not possible to establish whether or not P. coccineus subsp.darwinianus is the result of a cross between P. coccineus and P. vulgaris, although it was possible to establish that P. glabellus is a taxon only distantly related to other members of the complex.

STUDIES OF NUCLEAR DNA CONTENT
There is much variation in the reported DNA content of the Phaseolus species studied by different authors.The DNA content of cultivated P. vulgaris has been reported as being 1.56, 1.63, 1.69, 1.79 pg by Castagnaro et al. (1990), 2.7 pg by Bennett (1982) and 3.7 pg by Ayonoadu (1974), while that of the wild-type P. vulgaris var.aborigineus has been reported to be 1.71 pg by Castagnaro et al. (1990).These differences may be attributable to the source of the material, the type of control used or to errors inherent in the technique.Other species studied for their DNA content include P. coccineus, containing 3.5 pg according to Ayonoadu (1974) and 1.98 pg according to Castagnaro et al. (1990); P. lunatus with 2.5 pg (Ayonoadu, 1974); P. dumosus with 3.8 pg, and P. leucanthus with 3.3 pg (Ayonoadu, 1974).
The latter two species are probably P. coccineus subsp.darwinianus, since both names have always been nomenclaturally associated with this subspecies.
As is the case in cytogenetic studies, there are reports of species referred as Phaseolus when they actually belong to other genera, for instance, P. angularis is really Vigna angularis, with a DNA content of 2.8 pg, while both species P. geophilus (2.6 pg) and P. lathyroides (2.3 pg) belong to genus Macroptilium.Ayonoadu (1974) found a positive correlation between the nuclear DNA content and the nuclear volume, nucleolar and nuclear dry mass and total dry mass, i.e., high DNA content indicates high values for volume and dry mass parameters.Castagnaro et al. (1990), studying P. coccineus and several cultivars of P. vulgaris, along with P. vulgaris var.aborigineus, also found a positive correlation between seed weight and DNA content, with the exception of P. vulgaris var.aborigineus, which presented a negative correlation.Even so those authors conclude that varieties with a high DNA content are better adapted to cold or temperate regions, while those varieties with a lower DNA content are adapted to hot, dry environments.
We are now in the process of analyzing the DNA content of wild species of Phaseolus to determine if there is any relationship between DNA content and taxonomic relationships between the species and/or karyotype.
have pointed out that the main factors involved in the karyotypic evolution of the genus are pericentric and paracentric inversions, translocations and the loss or gain of chromatin.They have also proposed that the karyotype Cytogenetic studies in Phaseolus L. (Fabaceae)Pedro Mercado-Ruaro and Alfonso Delgado-Salinas Instituto de Biología, Universidad Nacional Autónoma deMéxico, Delegación Coyoacán,  04510 México, D.F., México.Send correspondence to P.M.-R.