Coix: An Unique and Remarkable Genus

The grass genus Coix of the monoecious tribe Maydeae is unique in having its pistillate spikelet enclosed in an indurated leaf sheath, which becomes stony-hard on seed formation called fruit case, false fruit, shell etc., and used as a bead in tropical Asia in tribal jewellery. Coix has a myriad uses as food, fodder, medicine etc.

Of the three conventionally recognized species of Coix, C. aquatica (2n=10) has large chromosomes and C. gigantea and C. lacrymajobi (job's tears) have small chromosomes (2n=20 and 2n: 4x=40 in the former and 2n=20 in the latter). They occur throughout South and South-East Asia, and C. lacrymajobi in all warmer parts of the world. The rest of the six species are endemic to restricted areas, and could be endangered (except C. puellarum). All species and varieties have hard fruit cases, except var. Ma-Yuen of C. lacrymajobi (adlay) which has soft shells, and is cultivated as a cereal in sub-himalayan slopes of India. Conventional species occur in aquatic (C. aquatica), mountainous terrain (C. gigantea) and dry (C. lacrymajobi) habitats, but they thrive well in any of these habitats by changing the reproductive process and altering the plant structure to be compatible with the habitat as adaptive measures. They are potentially perennial and occurrence of semigamy in C. aquatica is the first report in Poaceae.

Pachytene chromosomes are differentiated in Coix, exceptionally in a race of C. lacrymajobi, they are completely euchromatic, and in C. aquatica, they are the longest in the tribe. Intra-and interplant chromosome numerical mosaicism in C. aquatica and C. gigantea and translocation heterozygosity in C. aquatica have adaptive significance and help to maintain and enlarge cytogenetic diversity.

Genetically controlled bivalent pairing in aneuploids and polyploids results in racial differentiation and stabilization. Races with 2n=12 in C. aquatica and 2n = 2x=18, 22 and 2n=4x=40 in C. gigantea are well established.

2x = Two ancestral n=5 small chromosome forms by allopolyploidy yielded C. lacrymajobi and C. gigantea, the former has a stable genome, differentiated into six morphological varieties through accumulation of gene mutations. One of the two ancestors also gave rise to C. aquatica by accumulation of duplicate segments and heterochromatin resulting in its having longer chromosomes, which could be recognized at meiosis from the small chromosomes of the other two species in their interspecific hybrids, facilitating the study of inter-and intragenomic pairing in the haploid state. The two aneuploid species (2n=32) from Madhya Pradesh and West Bengal forming six large and 10 small bivalents at meiosis arose as allopolyploids with C. aquatica (n=6 race) as one parent, and C. lacrymajobi as the other parent in the former and C. gigantea in the latter.

C. ouwehandii, C. poilanei and C. puellarum arose as ecogeographic species from C. gigantea, C. aquatica and C. lacrymajobi var. monilifer in Northern Sumatra, Indonesia, in Laos, and in Myanmar, Malaysia and Indochina respectively. C. gasteenii originated as an alloploid of C. lacrymajobi (2n) and C. gigantea (2n) and flourished in Cape York Peninsula, Australia.

Coix is distinct from both American and Asiatic Maydeae and occupies an intermediate position between the two groups. Coix species serve as good class work material for teaching cytogenetics.