Chromosome characterization of the race Jala of maize
--A. Lòpez R., T.A. Kato Y. and F. Castillo G.
Jala is one of the most astonishing types of maize because of its ear size. Its geographic distribution is restricted to the small valley of Jala, located in the southern part of the state of Nayarit in Mexico; however, we do not know how variable it is across the valley regarding its chromosome knob constitution, since there was only information on six plants from a single collection: NAY72 (McClintock et al., Colegio de Postgraduados, Chapingo, Mexico, 1981). Based on two types of available collections, those recently collected (1988) by the Colegio de Postgraduados, and the old ones conserved in the CIMMYT germplasm bank which were collected between 1944 and 1968, a cytological study was carried out with two objectives: 1) to determine whether different populations of the race are relatively uniform or not; and 2) to determine if the race has changed its chromosome knob constitution with time.
Six accessions of the race Jala were studied: three collected by the Colegio de Postgraduados in 1988 (CP88-2, CP88-5 and CP88-6) from three different farmers in the valley, and three from the CIMMYT germplasm bank (NAY 6, NAY 53 and NAY 208) originally collected in the same place in 1944, 1952 and 1968, respectively. The chromosome knob constitution of 20 plants per accession was determined by examining pachytene meiotic microsporocytic cells with the usual propionic carmine squash method for 21 knob positions. Knob frequency, weighted by knob size, was subjected to principal component and cluster analyses by SAS (1985) procedures.
The cumulative variance accounted by the first two principal components was 65% and the dispersion of the characterized accessions determined by them is shown in Figure 1. An outstanding feature is the close grouping of the CP88 collections far from the NAY collections, of which NAY 6 is the most distant. This dispersion is due mainly to the higher positive values in the eigenvector of the first principal component, given by the frequencies of medium knobs in 1S2, 2S1, 2L1, 4L1, 6L2 and 7L1, of the small ones in 8L2 and 7S, of the large knobs in 2S1 and 5L1 and frequencies of the abnormal 10 chromosomes, all of them with higher frequency in the NAY collections, especially in NAY 6. The dispersion is also due to the high negative values in the eigenvector of this principal component associated with the frequencies of the small knobs in 2S1, 3L1, 5L1 and 6L3, of the large ones in 2L1 and 4L1, and of the B chromosomes, with higher frequencies in the CP88 collections. The second principal component provides distances that separate two collections from the third one in each of the groups CP88 and NAY, given by the higher negative values in the eigenvector, which are associated with the frequencies of large knobs in 1S2, 7L1, 8L1 and 9S, of the medium knob in 7S and of the small one in 1L1. The previously described dispersion is in close agreement with the dendrogram obtained from the cluster analysis shown in Figure 2. These results provide evidence that in spite of the restricted distribution, this race has some degree of variation among populations within the limits of the small valley of Jala.
If each of the four knob sizes in the 21 positions in the chromosomes of maize is considered as one allele in 21 corresponding loci, the average genetic diversity per collection was calculated, with the results shown in Table 1.
Table 1. Values of genetic diversity for six collections of the race
Jala of maize, obtained from the frequencies of four knob sizes in 21 chromosome
|Collection||Procedence||Date of Collection||Genetic Diversity||Average by Origin|
It can be observed that the values obtained for the CIMMYT collections are lower than those for the CP88 collections and of the former the lowest is for NAY 6 (collected in 1944), followed by NAY 53 (1952) and NAY 208 (1968). The lower diversity could be attributed to phenomena as those independently mentioned by Salhuana and by Crossa (CIMMYT Proc. Global Maize Germplasm Workshop, 29-38 and 159-154, 1988) of genetic drift that may occur during the seed increases by the germplasm banks due to the small size of seed samples used. Other factors that might have contributed to this differentiation are the occurrence of physical or biotic environmental modifications in the region during the time considered, the introgression of genes from introduced new varieties, the reduction in the crop farming area of this race, or because of changes in the selection criteria made by new generations of farmers.
Additionally, the chromosome constitution of the race Jala was analyzed with respect to 21 Mexican races whose data were taken from McClintock et al. (Colegio de Postgraduados, Chapingo, Mexico, 517 p., 1981) with the aim of ascertaining the relations among those 22 races, based on the same statistical methods. Race Jala (JL) was placed near the Tuxpeño complex: Tuxpeño, Vandeño and Celaya (TX, VA, CE) as shown in Figure 3, due mainly to the high frequencies of medium knobs in the long arms of chromosomes 2, 3, 4, 5, 7, 8, 9.
Figure 1. Dispersion of six maize Jala accessions, on the plane defined by the first two principal components, based on the weighted frequencies of four knob sizes in 21 positions in the chromosomes, and the presence of supernumerary elements.
Figure 2. Dendrogram obtained from the cluster analysis of weighted chromosome knob frequencies of six maize Jala accessions.
Figure 3. Clustering of 22 Mexican maize races in the plane determined by the first two PC, from the analysis of the weighted frequencies of four knob sizes in 21 chromosome positions, and those of supernumerary elements.
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