GISH affinities between subspecies of Zea mays --Gonzalez, G*, Confalonieri, V*, Comas, C*, Naranjo, CA, Poggio, L*
*also affiliated with Dpto. de Ecología, Genética y Evolución, FCEN, UBA
Zea mays(L.) is composed of four subspecies (Doebley, Maydica 35:143-150, 1990): Zea mays ssp. mays (maize), Zea mays ssp. mexicana, Zea mays ssp. parviglumis and Zea mays ssp. huehuetenanguensis. The last two taxa have a disjunct geographic distribution (Doebley and Iltis, Amer. J. Bot 67:982-993, 1980; Iltis and Doebley, Amer. J. Bot 67 (6):994-1004, 1980), and were considered as varieties of Zea mays ssp. parviglumis until 1990, when Doebley separated them as subspecies.

Isoenzymatic and molecular marker studies (Doebley et al., Econ. Bot. 42:120-131, 1989; Hilton and Gaut, Genetics 150:863-872, 1998; Matsuoka et al., Proc. Natl. Acad. Sci. USA, 99 (9):6080-6084, 2002) showed that Zea mays ssp. mays would have more affinity with the annual teosinte Zea mays ssp. parviglumis. Nevertheless, in a previous study (Gonzalez et al., MNL 75:36, 2001) we demonstrated that there are divergent chromosome regions between these two taxa, by means of GISH (Genomic In Situ Hybridization) experiments on chromosome preparations of Zea mays ssp. parviglumis. The present work intends to further investigate the genomic homology between Zea mays ssp. mays (native race of Argentina, cult. 6484, Instituto Fitotécnico Santa Catalina - IFSC) and two of the other subspecies: Zea mays ssp. parviglumis (Balsas, cult. 6836, IFSC, provided by CIMMYT) and Zea mays ssp. huehuetenanguensis (cultivated at IFSC, provided by CIMMYT). The study was performed by means of GISH, without applying a blocking procedure, and using chromosomes of Zea mays ssp. mays as the target of hybridization.

Chromosome preparations and GISH were carried out according to Poggio et al. (Genome 42:993-1000, 1999).

When maize chromosomes were hybridized with labelled genomic DNA from Zea mays ssp. parviglumis, we observed hybridization signals of variable intensities throughout all of the chromosomes, with the exception of the centromeric and nucleolar organizer regions, where the hybridization is weak or absent, indicating that these are zones of less homology between both taxa. Then, when genomic DNA of Zea mays ssp. huehuetenanguensis was used as a probe on maize chromosomes, we observed that in addition to the absence of hybridization signals in the centromeric and nucleolar organizer regions, many other chromosome regions were unlabelled. In fact, at least 3 chromosomal pairs display similar patterns of hybridization, showing one chromosome arm highly labelled and the other one unlabelled or weakly hybridized. These results indicate an important divergence between both subspecies.

We conclude that a greater general homology is evident between maize and Zea mays ssp. parviglumis than with the other subspecies analyzed. These results are in agreement with previous molecular studies that propose Zea mays ssp. parviglumis as the closest related taxa of maize, although they do not corroborate the ancestoródescendant relationship proposed by some authors cited above. On the other hand, the degree of divergence observed gives further support to the last subdivision of the Zea mays species made by Doebley (1990).

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