Chromosomal Studies in the Egyptian Flora V. Chromosomal relationships in the genus Astragalus L. (Fabaceae) and their taxonomic inference.

Abstract

The karyotypes of 17 taxa representing eleven species of Astragalus have been described and their taxonomic inferences are discussed. A diploid chromosome number of 2n =16, with a basic number of x = 8, was recorded in A. eremophilus, two samples of A. peregrinus, A. sieberi, A. sinaicus and A. trigonus. Polyploid numbers, also based on x = 8, were scored, 2n = 32 in A. hamosus and 2n = 40 in A. hamosus v. buceras. Meanwhile, a diploid number of 2n = 14, with a basic number of x = 7 was found in three samples of A. annularis and three varieties of A. tribuloids. A. boeticus was found cytologically distinguished with 2n = 30, and A. tenuirugis with 2n = 24, while A. trimestris has 2n = 12 and x = 6. The chromosomes in the genus Astragalus are relatively samll, among the examined species the largest were found in A. annularis (mean length = 1.7-1.75 gm) and the smallest were recorded in A. boeticus (mean length = 0.95 μm). In general the karyotype of the genus show a relatively high degree of symmetry as reflected by r-value and the TF%. Infra and inter-specific variations, in these criteria, have been recorded among the studied taxa.The preponderance of species with a basic number of x = 8 among the studied taxa agree with the assumption that this number is the primary number in Astragalus, x = 7 and x = 6 could have been evolved by aneuploid loss of chromosomes. This situation is only found in the Old World species of the genus. In the New World species, however basic numbers of x =11, 12, 13, 14 and 15 have been often encountered, these numbers may have been evolved from x = 8 through polyploidy followed by loss of 1-4 chromosomes. Discussion of chromosomal relationships in this large and complex genus chould take into account that only about 25% of the species have been cytologically examined. Moreover chromosomal changes should not be discussed independent of other criteria particularly those of morphology. Among the examined taxa chromosomal features provide evidence for the differentiation of some taxa which share mophological similarities, meanwhile some other taxonomically related species were found to have similar karyotype features.