Chromosomal basis of evolution in the genus Coix L. (Maydeae): a critical appraisal.

Among the nine species of the genus Coix, C. lacrymajobi L. and C. gigantea Koen. have 2n = 20 small chromosomes and C. aquatica Roxb. 2n = 10 large chromosomes, and are widely distributed in South and South East Asia, the first is cultivated and spread to all warmer parts of the world. The other species have restricted localized distribution. C. aquatica originated from a hypothetical diploid ancestor with 2n = 10 small chromosomes, which became longer through accumulation of duplicate segments and heterochromatin and adapted to wet habitats. C. lacrymajobi and C. gigantea originated through the same two small chromosome diploid ancestors, one of which may be common to C. aquatica, through allopolyploidy and adapted to drier conditions. All the three species can survive equally well in wet and dry localities. Aneuploid and polyploid races occurred through chromosome nondisjunction and genome doubling in C. gigantea and C. aquatica, generation of alien-addition and substitution chromosome races appears to be the further course of evolution. Six morphological varieties were differentiated in C. lacrymajobi through accumulation of gene mutations, and chromosome structural changes gave rise to a completely euchromatic chromosome race. Two aneuploid species with 2n = 32 chromosomes, both large and small, must have originated through allopolyploidy, with C. aquatica ( n = 6) race as one of the parents in both, C. lacrymajobi and C. gigantea ( n = 10) the second parent in Madhya Pradesh form and West Bengal form respectively. From comparison of morphology and their distribution, C. puellarum Balansa, C. ouwehandii Koord. and C. poilanei Mimeur are speculated to have originated from the three established species, through chromosomal changes and gene mutations and adapted to restricted localized areas, and C. gasteenii Simon an allopolyploid form of C. lacrymajobi and C. gigantea. A schematic diagram is presented depicting the origin, evolution and interrelationships in the genus. [ABSTRACT FROM AUTHOR]