Isozyme diversity in Zea and related genera

--S. K. Katiyar and J. K. S. Sachan

Unaccountable numbers of genetic and biochemical studies have allowed a still closer examination of evolutionary relationships within Zea, but almost all of the studies were limited to American genera of Maydeae; biosystematic studies on Asiatic Maydeae have never been considered seriously.

During the present investigation the members of both American and Asiatic genera of the tribe Maydeae, comprised of 18 distinct collections of Coix spp. (including both Coix aquatica (2n=10) and C. lacryma jobi (2n=20)), 7 of Chionachne, 7 of teosinte (including Zea mexicana, Z. diploperennis, Z. luxurians and Z. mays spp. parviglumis) and a cultivar of maize (Zea mays L.) were studied for their pollen isozyme banding patterns produced by PAGE of esterase, peroxidase, malate dehydrogenase, acid phosphatase, superoxide dismutase, catalase and glutamate dehydrogenase.

The genetic distances between different collections within the genus and between the genera were calculated from the average similarity indices. The close clustering and resemblances between four Zea mexicana collections were noticed. Among the remaining three species, Zea mays ssp. parviglumis was found to be closer to Z. diploperennis than to Z. luxurians, Z. mexicana was found to be closer to maize than Z. diploperennis and Z. mays ssp. parviglumis. Zea luxurians was found to be distantly related to all the species of teosinte and maize. The results of the present study suggest that maize and teosinte are closely related, but at the same time there is a sizable dissimilarity within the teosintes. The electrophoretic movements of different isozymes in Z. diploperennis, Z. mexicana, Z. luxurians and Z. mays ssp. parviglumis, suggest that genetic differences among these species are greater than evident by morphology. All the Chionachne collections showed close resemblance to each other irrespective of their place of origin. In the same way all 18 collections of Coix clustered separately though the 2 species Coix aquatica and C. lacryma jobi are distinctly related.

On the basis of the intergeneric electrophoretic affinities the following similarity hierarchy was obtained

Maize : Teosinte Chionachne Coix

Teosinte : Maize Chionachne Coix

Chionachne : Coix Teosinte Maize

Coix : Chionachne Teosinte Maize

Among the tribe Maydeae genetic distance (GD) was the least (0.309) between maize and teosinte, while Chionachne and Coix were farther from both maize and teosinte. These results strongly suggest that Asiatic genera are quite distinct from the American ones. Even both members of the Asiatic Maydeae, Coix and Chionachne, were also found to be distantly related (GD=0.645) to each other.

Table. Genetic distances among different genera of the tribe Maydeae.
 
  Maize Teosinte Chionachne Coix
Maize 0.000      
Teosinte 0.309 0.000    
Chionachne 0.781 0.740 0.000  
Coix 0.853 0.766 0.645 0.000


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