Pakistan Journal of Biological Sciences

Year: 2007 | Volume: 10 | Issue: 1 | Page No.: 49-56
DOI: 10.3923/pjbs.2007.49.56

Karyotype Analysis and Chromosome Evolution in Species of Lathyrus (Fabaceae)

Salwa Fahmy Badr

Abstract: The karyotypes of 21 accessions of Lathyrus L. belonging to four sections were investigated. Although all the species have a chromosome number of 2n = 14, they could be differentiated by their karyotype formula and quantitative parameters of the karyotypes. Phenetic distance showed that in spite of the differences observed among entitied, they can be grouped in clusters that coincide with the taxonomic sections established by Kupicha and with the life cycle of the species. The section Clymenum can be distinguished by the presence of a subtelocentric pair. From an evolutionary point of view, variation in genome size, however, is congruent with morphological variation as well as with the life cycle.

Keywords: Lathyrus, Fabaceae, chromosome evolution and Karyotype

INTRODUCTION

Lathyrus is distributed throughout the temperate regions of the Northern Hemisphere with 52 species in Europe, 30 species in North America, 78 species in Asia and 24 species extending into tropical East Africa and 24 species into temperate South America (Kupicha, 1983; Allkin et al., 1985; Goyder, 1986). The genus Lathyrus (Fabaceae, Papilionoidae, Vicieae) consists of 13 sections and comprises about 150-160 species of annual or perennial herbs with erect or, more usually, climbing and sprawling habit, which are predominantly self-pollinating (Kupicha, 1983). The main center of diversity is the eastern Mediterranean region, with two smaller centers in North and South America (Kupicha, 1983; Allkin et al., 1985; Simola, 1986). Lathyrus species occur in a diversity of habitats, including open woods, forest margins, meadows, pastures, fields, slopes, marshes, seashores, sand dunes and roadsides. Lathyrus exhibits a typical bee-pollinated papilionoid flower, which may be yellow, orange, red, purple, violet, bluish, or white (Conny et al., 1998). There are many species cultivated for forage and human food: L. sativus (grass pea), L. hirsutus (rough pea), L. cicera (flatpodded vetchling), L. odoratus (sweet pea), L. ochrus (ochrus), L. sylvestris (flat pea).

Cytological investigations have shown that all species were diploid with 2n = 14 chromosomes, the basic chromosome number of x = 7 is constant throughout the genus and that most of the species are diploid, with polyploids as rare exceptions (Senn, 1938; Yamamoto et al., 1984; Broich, 1989; Battistin and Fernández, 1994; Klamt and Schifino-Wittmann, 2000; Seijo and Fernández, 2001). Despite this stability in chromosome number, large variations in chromosome size have played an important role in the evolution of Lathyrus species, which are associated with a fourfold variation in 2C nuclear DNA amount (Narayan and Durrant, 1983). The species show marked similarity in their chromosome shapes and karyotype arrangements within complements. The amounts of nuclear DNA within this genus are discontinuously distributed. Many karyotypic studies have been performed on Old World members of Lathyrus (Lavania and Sharma, 1980; Yamamoto et al., 1984; Sahin et al., 2000), but there is a paucity of data for American species, with the karyotypes of only five South American entities described so far (Battistin and Fernández, 1994; Klamt and Schifino-Wittmann, 2000). From the available information, a number of conflicting observations have arisen. Some authors claim that, in addition to the numerical constancy, Lathyrus species display morphological uniformity of chromosomes and homogeneous karyotype arrangement (Lavania and Sharma, 1980; Narayan and Durrant, 1983; Klamt and Schifino-Wittmann, 2000). However, others have found enough interspecific karyotype differences to allow species characterization (Yamamoto et al., 1984; Murray et al., 1992a; Battistin and Fernández, 1994). Such discrepancy was also observed at the infraspecific level, mainly in the widely studied L. odoratus L. and L. sativus (Bhattacharjee, 1954; Sharma and Datta, 1959; Verma and Ohri, 1979; Murray et al., 1992b). From a karyosystematic point of view, Yamamoto et al. (1984) have noted that Old World species could be grouped according to their karyotype morphology and that some of them were coincident with the taxonomic sections proposed by Davis (1970). However, these authors did not propose any relationship, either among groups or considering the world infrageneric classification proposed by Kupicha (1983), Przyblska et al. (1999), Seijo and Fernández (2003) and Wong et al. (2006).

Genetic relationships within species of the genus Lathyrus were reported by Datta (1955) and Chaudhuri (1966). In addition to this numerical chromosome constancy species display uniformity in chromosome morphology (all chromosomes are metacentric or submetacentric). Evolution, nevertheless, has resulted in a large increase in chromosome size (Narayan and McIntyre, 1989). In general, the total length of somatic chromosome set was longer in perennial species than in annual ones as reported by Rees and Hazarika (1969). Species of the section Clymenus, Nissolia and Aphaca have chromosome complements larger than those in the section Cicercula. The main karyotypic difference between species of the genus Lathyrus involves the shape of satellite chromosome (Yamamoto et al., 1984). The divergence and evolution within this genus is accompanied by a 3-fold increase in chromosome size which is directly correlated with 4-fold increase in their nuclear DNA amounts. Comparisons of total DNA amounts showed discontinuity in the variation among species of this genus (Narayan, 1982).

It has been reported that inferageneric Lathyrus classification has varied markedly during its history and classifications based on morphology are problematic due to homoplasy and lack of diagnostic characters (Conny et al., 1998). Thus, the present investigation is concerned with karyotype analysis and idiogram of the genus Lathyrus to clarify the taxonomic relationships among species of this genus and to examine the pattern of chromosome variation in relation to the taxonomic position and the life cycle of the taxa.

MATERIALS AND METHODS

In the present work, cytological study was carried out on 21 accessions samples obtained as a donation from Western regional plant introduction station 59 Johnson Hall Pullman (PI), Washington 99164 and ICARDA Genetic Resource (IG). The accessions represent 13 species belonging to four sections. For cytological preparations, seeds were germinated on moist filter paper in petri dishes at room temperature (18-22 C).1-2 cm long roots were detached and pretreated with a saturated solution of 1,4-dichlorobenzene for 3-4 h. Roots were then washed briefly in water and fixed in a mixture of 3:1 (v/v) ethyl alcohol: glacial acetic acid for 24 h and kept in a refrigerator until use.

Cytological preparation were carried out using the Feulgen squash technique for Feulgen staining root tips were hydrolyzed in 1N HCl at 60°C for 8-10 min, washed in distilled water and stained in Leuco-basic fuchsin for at least 1 h. The terminal 1-2 mm of the root tips were squashed in a drop of 45% acetic acid on a clean slide. Cover slips were separated by the freeze-drying method. samples were then dehydrated in absolute ethanol for 2-3 min and made as permanent preparation by mounting in Depex slides were allowed to dry at room temperature for few days. Cells with good spreading of chromosomes were photographed using a zeiss Ultraphoto microscope equipped with automatic camera. The nomenclature used for the description of the chromosome morphology is that proposed by Levan et al. (1965). The abbreviations m, sm and st designate metacentric, submetacentric and subtelocentric chromosomes, respectively. Idiograms were drawn based on mean centromeric index and arranged in order of decreasing size.

DATA ANALYSIS

For the numerical characterization of the karyotypes the following parameters were calculated: (1) total chromosome length of the haploid complement (TCL); (2) mean Chromosome Length (CL); (3) mean Centromeric Index (CI); Comparisons of chromosome morphological features were made by arranging the chromosomes of each karyotype in pairs in order of their arm ratio and length as determined from the photographic prints. An idiogram for each sample was constructed using the total length of each pair of homologous chromosomes to represent the haploid chromosome number. The relative position of the centromere and their variation within the karyotype has been expressed. A cluster analysis of the karyotype data was coned out to examine karyotype similarity among species and sections. A data matrix 21 OTUs (operational taxonomic units) x 7 variables was constructed. The TCL, CI, number of m, sm and st chromosomes as well as the life cycle were considered. The SYSTAT ver. 7 program was used to standardize the data matrix, to calculate the average taxonomic distance and to generate a phenogram. Clustering was performed using the unweighted pair-group method (UPGMA).

RESULTS

All the studied samples have 2n = 14 chromosomes. Karyotype formulae and parameters for the studied species are summarized in Table 1. Figure 1 shows the mitotic metapbases and Fig. 2 the respective idiograms.

As a whole, karyotypes of the analyzed species, in the four sections, have a predominance of m chromosomes. The most common formula among section Cicercula species is 12 m+2 sm (five species), followed by 14 m (two species). In the section Lathyrus the dominance formula is 8 m+6 sm (four species) and the second formula is 10 m+4 sm (three species).

The section Clymenum characterized by the presence of two subtelcentric chromosomes and different number of m and sm chromosomes. Section Aphaca has a formula 10 m+4 sm. Considering the life cycle, perennial species have a greater TCL than the annual. Considering the mean value of TCL and CI among the studied section, the section Lathyrus has the longest Total Chromosome Length (TCL) and but the centromeric index is almost the lower than those in section Cicercula. The idiograms as well as the parameters presented in this paper represent the means of the populations analyzed for each species. However, at the interspecific level, TCL and CI significantly differentiate some species (Table 1).

The relationship between TCL and CI of each species is plotted in Fig. 3. ANOVA of TCL discriminated among species of the four sections analyzed (F = 20.140, p<0.01) and the CI (F = 4.201, p<0.0021).

The UPGMA phenogram constracted on the basis of karyotype similarities shows two major clusters (Fig. 4). The first cluster is comprised the two samples of L. sylvestris in section Lathyrus, which are characterized by the highest CI, TCL and perennial species and the second cluster include the rest of studied samples. This cluster comprised two major groups, the first one includes the samples of section Clymenum, which characterized by the presence of two subtelcentric and the second groups separated into three subgroups. The samples of L. cicera, L. inconspicus and L. blepharicarpus, which are belonging to section Cicercula are delimited in one subgroup. The second subgroup includes the samples of L hirsutus. The species L. annus, L. sphaercus, L. latifolius, L. aphaca and L. marmoratus are delimited in the last subgroup. L. marmoratus is delimited with L. latifolius due to the similarities in the CI and life cycle.

DISCUSSION

The results of this study revealed a detailed picture of the chromosome features of some Lathyrus species belonging to four sections and of their pattern of variation in relation to their systematic position and life cycle. Like most species of Lathyrus, all the species analyzed here were diploids with 2n = 14. The chromosome numbers of the species agree with those published previously (Senn, 1938; Lavania and Sharma, 1980; Yamamoto et al., 1984; Battistin and Feranández, 1994; Klamt and Schifino-Wittmann, 2000). In addition to this numerical chromosome constancy species displayed uniformity in chromosome morphology (all chromosomes were either metacentric or submetacentric) and in karotype arrangements. Evolution, nevertheless, has resulted in a large increase in chromosome size which is associated with a five-fold increase in 2C nuclear DNA amounts (Rees and Hazarika, 1969). Cytological and molecular investigations into the organization of chromosomes and the composition and distribution of nuclear DNA within and between chromosome complements have given evidence that there are strong constraints controlling evolutionary changes in genome organization (Narayan, 1982; Narayan and Durrant, 1983; Narayan and Rees, 1976).

L. clymenum and L. ochrus belonging to the section Clymenum, these two species are separate in cluster tree from L. articulatus. The lower degree of similarity among them may be attributed to a considerable difference in karyotype, including differences in chromosome length between individual chromosomes of both species and also difference in some morphological variations such as pod shape and structure, these agree with protein bands reported by El-Shanshoury (1997).

Karyotype formulae and quantitative analysis have a great uniformity among populations of any species, except those that correspond to different taxonomic varieties. These results support the hypothesis that claims infraspecific stability of karyotypes in Lathyrus species (Murray et al., 1992b). At the interspecific level, quantitative and qualitative data allowed the differentiation of several of the taxa studied. Among species of section Lathyrus and Cicercula, the most variable characters were the number of m chromosomes, as well as the karyotype formulae were more similar, but species still could be differentiated mainly by the number and type of chromosomes. These facts show that the karyotypes of Lathyrus species are not as fully constant as has been postulated (Narayan and Durrant, 1983; Klamt and Schifino-Wittmann, 2000) and that entities may be characterized by their chromosome features as was suggested by other authors (Yamamoto et al., 1984; Murray et al., 1992a; Battistin and Fernández, 1994).

In relation to the genome size variation, there are differences in the chromosome length among the studied taxa. These differences among complement length of diploid species are in accordance with that cited for nuclear DNA amounts of Old World species of Lathyrus (Rees and Hazarika, 1969; Narayan, 1982) and support the statement that variation in genome size is, perhaps, one of the more striking changes that have occurred during the divergence and evolution of the chromosome complements of this genus (Narayen and Rees, 1976). As a whole, Lathyrus is characterized by symmetrical karyotypes, with a predominance of sm chromosomes (Yamamoto et al., 1984), but the sections under this study have a predominance of m chromosomes and the presence of a st pair is characterized to section Clymenum. In spite of the observed interspecific variation, the bulk of karyotype data-formula, TCL and CI showed a conservative tendency toward the maintenance of the general structure of the karyotypes among different clusters of species, in accordance with observations by Yamamoto et al. (1984). Analysis of karyotype formulae showed that, in general, species of section Clymenum have a pair of st. chromosome; they differ from the other sections, Lathyrus, Aphaca and Cicercula. Therefore, karyotype features may become good taxonomic characters to define members of Clymenum.

The bulk of available karyotype data showed that most of karyotype groups found in Lathyrus could be related either to different sections sensu Kupicha (1983) or to the life cycle of the entities. The phenctic analyses of karyotype characters support this postulate. At the interspecific level, within section Lathyrus and Cicercula, some species can be distinguished clearly by their karyotype formulae and when quantitative karyotype data are added, the majority of entities can be differentiated. The constancy in chromosome number observed in the species studied here and in those cited in the literature (Senn, 1938; Battistin and Fernández, 1994; Klamt and Schifino-Wittmann, 2000; Seijo and Fernández, 2003) indicates that numerical changes have not been important in the evolution of South American species, as noted for most of the entities of Lathyrus (Hitchcock, 1952; Yamamoto et al., 1984; Sahin et al., 2000). However, this constancy differs from the situation described for North America, where several endemic polyploid species were found, so that North America was considered as a center of polyploid origin for Lathyrus (Broich, 1989).

Present findings that TCL varies without significant changes in karyotype formula, as seen among annual and perennial species of sections Cicercula and Lathyrus, suggest that changes in genome size may have been nonrandom and that the variation in DNA amounts is equally distributed among all chromosomes of the complements. These observations agree with those cited for Old World species of Lathyrus, in which variation of genome size was attributed to proportional distribution of mainly moderately repetitive DNA throughout the complement (Narayan and Durrant, 1983). Data obtained from banding patterns also support the nonrandomness of genomic change in Lathyrus because bands with similar base composition tend to have equilocal disposition in the karyotypes (Unal et al., 1995; Seijo, 2002). This pattern of evolution at molecular and subchromosomal levels suggests that species within each group evolved in a concerted fashion, maintaining the karyotype morphology.

The reduction of genome size that accompanied the evolutionary change from perennial to annual in sections Lathyrus and Cicercula species coincides with different reports on angiosperm groups (Price and Baehmanm, 1976; Greilhuber and Ehrendorfr 1988; Seijo and Fernández, 2003). Moreover, annual species of this section, in addition to having lower TCL, present smaller pollen grains and lighter seed than perennials (Seijo, 2002). These observations are in agreement with a considerable amount of data that show that the size of reproductive organs may be related to the genome size (Choi, 1971; Chung et al., 1998); as was postulated in the nucleotype hypothesis (Bennett, 1972).

ACKNOWLEDGMENT

The author is most grateful to Prof. R.H. Sammour a Prof. of Molecular Systematic, Tanta University, Egypt, for kind supply of seed materials and his helping in computer analyses and revising the manuscript.

REFERENCES