PERGAMINO, ARGENTINA
Mejoramiento Genético de Maíz-EEA-INTA

Analysis of heterotic maize (Zea mays L.) populations using molecular markers

— Morales Yokobori, ML; Decker, V; G.y Ornella, LA

Argentina is one of the main corn producer countries in the world. In fact the volume of its exports ranks second only to the United States. The average annual corn production estimate for Argentina is 14 million tons, of which around 50% are exported, and the remainder are used for food, feed, seed and industrial purposes.

A common strategy of breeding programs in Argentina is to take advantage of the strength of the argentine Orange Flint vs. U.S. Yellow Dent germplasm heterotic pattern1. Flint hybrids are appreciated in the market not only because of the hardness of the endosperm which makes the kernel less susceptible for breakage and more suitable for dry milling industry, but also for their greater resistance to drought stress, MRCV (Maize Río Cuarto Virus) and ear rots(1). In general, chemical composition indicates flint kernels have a larger biological value when compared to dent kernels(2). Despite breeding efforts, flint hybrids still show average grain yields, behind those of flint × dent and dent hybrids, across a range of environmental conditions. Thus, knowledge of genetic diversity and relationships among flint inbreds lines would help to reduce genetic vulnerability and broaden the crop genetic base in national improvements programs, allowing the planning of hybrid crosses and assignment of new inbreds to heterotic groups previously established.

In this report we present the results of the molecular characterization of 20 microsatellite loci evenly distributed in the genome of 26 inbred lines. All lines except one (B73) were developed by INTA (Instituto Nacional of Tecnologia Agropecuaria) from different sources (mainly landraces) and belong to the argentine Orange Flint heterotic group(1).

Amplifications were carried out using 10–20 ng of DNA template and a “touchdown PCR” with annealing temperatures varying from 65°C to 60°C. Amplification products were separated on 6%(W/V) polyacrilamide denaturing gels and were detected by silver staining (Silver sequence Promega Biotech, Madison, WI)(3). All PCR amplifications and gel runs were made in duplicate for each inbred.

Results in Table 1 show the number of alleles observed for each locus studied and its PIC (polymorphism information content). Missing data indicates the number of inbred without visible bands. The number of alleles/locus ranged from 2 to 14 with a mean of 5.9. Polymorphism information content was estimated at each locus as a measure of genetic variation by the formula: PIC = 1 - ∑p2i , where pi is the frequency of the i allele. PIC’s values ranged from 0.41 to 0.86 with a mean of 0.67.

Presence or absence of each single fragment was coded as one or zero, respectively, in a binary data matrix. Jaccard’s coefficient was selected to construct the similarity matrix. Cluster analysis was performed by the unweighted pair-group method analysis (UPGMA) method and the Infostat/P computer program v.1.6 (Grupo Infostat, FCA Universidad Nacional de Cordoba). The dendogram in Figure 1 shows the genetic relationships based on SSR analysis.

In general, the grouping agreed with the pedigree information of the lines, although some discrepancies were observed. However, the cophenetic value of 0.547 indicates a low-moderate fit of the dendogram with the similarity matrix generated with SSR data.

Results confirm microsatellites as an excellent complement to morphological and other conventional markers that are currently used to obtain plant variety protection for new maize lines (and eventually hybrids). However, their utility to assign new inbreds to heterotic groups remains to be explored.

References

Table 1. Position, number of alleles and PIC values obtained for each SSR locus

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