Pachytene chromosome morphology of Coix aquatica and its comparison with Zea chromosomes
--S. K. Katiyar and J. K. S. Sachan
For the first time detailed cytological studies in terms of pachytene chromosome lengths, total genomic length, relative chromosome length, arm ratio and centromeric index, distribution of euchromatic and heterochromatic regions and presence or absence of heterochromatic knobs in 26 distinct collections of Coix spp. belonging to different geographical regions of the country were carried out, keeping in view that no systematic attempts have been made to study pachytene chromosomes of C. aquatica and karyotypic differences in Coix and Zea chromosomes, as both belong to the same tribe, Maydeae. Six collections of maize of both Indian and American origin, and 3 species of teosinte viz. Z. diploperennis, Z. luxurians and Z. mays ssp. parviglumis were also included in the study for comparison and establishing the interrelationship of Coix with them.
The relative chromosome lengths among 26 collections of Coix aquatica (2n=10) studied ranged from 13.55 to 28.28, whereas, arm ratios varied in a very narrow range of 1.07 to 2.30. The relative lengths of chromosomes 1, 2, 3, 4 and 5 of Coix at pachytene ranged from 22.35-36.96, 19.96-28.61, 13.68-21.37, 11.47-20.11 and 8.81-17.52, with an overall mean relative chromosome length of 28.28, 22.74, 18.59, 16.80 and 13.55, respectively. Either chromosome 1 and 2 or 1 and 3 were found to be satellited. Chromosome 1 is metacentric (CI=48.14), chromosomes 2 (CI=41.18) and 3 (CI=40.57) are submetacentrics, whereas, chromosomes 4 (CI=30.24) and 5 (CI=33.95) are acrocentrics. The essential uniformity in the gross features of the pachytene karyotype of C. aquatica, except for heterochromatin distribution pattern, are clearly evident by comparative karyomorphological study of different collections. There is a gradual decrease of lengths from the longest to the shortest chromosome with no abrupt change in size, which shows that the C. aquatica karyotype is symmetrical. The positions of heterochromatic blocks on each chromosome vary from collection to collection, however, some positions are stable while others undergo rapid changes producing polymorphism at a particular position between different collections.
The genome of Coix aquatica (2n=10) at pachytene is about 1.5 times longer than that of Zea and the chromosomes themselves are quite dissimilar in morphology to those of Zea. Chromosomes of C. aquatica are differentiated, showing differentially stained eupycnotic and heteropycnotic regions. They are also devoid of knobs. Maize and teosinte chromosomes do not exhibit similar differential staining and they possess knobs. Karyomorphological comparison (Table 1) thus does not reveal common features between the genomes of Coix and Zea, and it seems that the genomes present are quite different.
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