Cytological and Karyotypic Studies in Four Beta Species

Abstract

The karyotype and its components have been used to decipher karyo-evolutionary trends within taxa. With this in mind, chromosomal characteristics of four species of the genus Beta were studied to find karyotype relation­ ships, vis-a-vis meiotic features, and to ascertain the feasibility of using these species for development of inter­ specific hybrids and polyploids in sugarbeet (B. vulgaris L.) breeding programs in subtropical India. Four species of Beta were studied; B. vulgaris L., B. vulgaris ssp. maritima, B. vulgaris ssp. oriel/talis, and B. lomantogona; all with chromosome numbers of2n=18. Their karyotypes were generally asymmetric. Total haploid chromatin length ranged from 17.92 to 24.17 ~m, whereas individual chromosome size ranged from 1.47 to 3.15 ~m. According to Stebbin's classification of asymmetry, these species ranged from 2A to 4A, thereby confirming an evolution­ ary trend among the karyotypes. The karyotype of B. vul­ garis L. var LS-6 was most advanced and was classified as 4A. The karyotypes of B. vulgaris ssp. maritima and ssp. orientalis were classified as 3A. To further quantify gradations within a class of asymmetry, a chromosome Dispersion Index (DI) was calculated. A DI value of 0.479 for B. vulgaris L. var LS-6 confirmed its high degree of karyotypic specialization. Meiotically, all the species formed predominantly open bivalents with distal chiasma localization. Chiasma formation per bivalent decreased as the length of pairing blocks increased. This suggested a species specific chromosome condensation gradient because sugarbeet karyotypes are relatively constant during somatic and meiotic phases.