S. K. Vasal

Use of SSRs for establishing heterotic groups in subtropical maize

Abstract Heterotic groups and patterns are of fundamental importance in hybrid breeding. The objectives of our research were to: (1) investigate the relationship of simple sequence repeats (SSR) based genetic distances between populations and panmictic midparent heterosis (PMPH) in a broad range of CIMMYT maize germplasm, (2) evaluate the usefulness of SSR markers for defining heterotic groups and patterns in subtropical germplasm, and (3) examine applications of SSR markers for broadening heterotic groups by systematic introgression of other germplasm. Published data of two diallels and one factorial evaluated for grain yield were re-analyzed to calculate the PMPH in population hybrids. Additionally, 20 pools and populations widely used in CIMMYTs breeding program were assayed with 83 SSR markers covering the entire maize genome. Correlations of squared modified Rogers distance (MRD2) and PMPH were mostly positive and significant, but adaption problems caused deviations in some cases. For intermediate- and early-maturity subtropical germplasm, two heterotic groups could be suggested consisting of a flint and dent composite. We concluded that the relationships between the populations obtained by SSR analyses are in excellent agreement with pedigree information. SSR markers are a valuable complementation to field trials for identifying heterotic groups and can be used to introgress exotic germplasm systematically.

Genetic diversity of maize inbred lines in relation to downy mildew

A major emphasis in maize breeding in Asian countries has been the improvement for resistance to downy mildew, a serious disease that causes significant yield losses. A total of 102 inbred lines, including lines from Asian breeding programs, Mexico, USA and Germany, were analyzed with 76 SSR markers to measure diversity and investigate the effect of selection for downy mildew resistance. A mean polymorphism information content of 0.59, with a range of 0.14 to 0.83, was observed. Diversity at the gene level showed an average of 5.4 alleles per locus and a range of two to 16 alleles per locus, with a total of 409 alleles. About half of the alleles in the Asian lines had frequencies of 0.10 or less, and only 2% had frequencies > 0.80, indicating the presence of many alleles, and thus a high level of diversity. Some of the high-frequency alleles were in chromosomal regions associated with disease resistance. However, the frequencies of alleles in three SSR loci that are linked to a QTL for resistance to downy mildews in Asia were not significantly different in the subtropical/tropical Asian lines as compared to all the lines in the study. Lines from the US, Germany, and China, comprised three clusters of temperate maize(GS = 0.31), while those from India, Indonesia, Philippines, Thailand, Vietnam and CIMMYT comprised seven indistinct clusters of subtropical and subtropical maize (GS = 0.29). We conclude that maize breeding activity in Asia has not caused a decline in the overall amount of diversity in the region.

Improvement in selfed and random-mated generations of four subtropical maize populations through S3 recurrent selection

SummaryIn maize (Zea mays L.) source germplasm having tolerance to inbreeding is needed for the derivation of productive inbred lines. The germplasm with non-temperature adaptation, generally suffer from large inbreeding depression (ID). the objective of the present study was to improve four white subtropical populations for tolerance to ID through S3 recurrent selection. Two cycles of selection were conducted and the effect of selection was evaluated in the first selfed (S1) and random0mated (Synthetic-2=Syn-2) generations. To complete each cycle. 200 to 250 S3 progenies were evaluated and 15 to 20 superior ones selected for recombination in each population. Selection was practiced for high grain yield, vigour and other agronomie traits. The S1 and Syn-2 generations of the original (C0) and improved (C1, C2) populations were evaluated in a split-split plot design, with populations in main plots, generations in subplots and cycles in sub-subplots. In all populations, S1 and Syn-2 generations of C2 were significantly higher yielding than the C0 and the linear gain per cycle varied from 16.3 to 28.8% in S1 generation and 5.5 to 10.7% in Syn-2 generation. Selection reduced the anthesis-silking interval but slightly increased plant height, grain moisture and days to silk. An increase in plant height was expected because of emphasis on vigour during selection. For grain yield. ID decreased from 48.4% in C0 populations to 37.9% in C2, indicating an improved tolerance to inbreeding. Further, the selection accumulated favourable alleles; and Pop. 42 seemed to be the best source populations for deriving high yielding inbred lines. It may be concluded that S3 recurrent selection was effective in improving grain yield and the improved populations would provide better germplasm sources for the derivation of productive inbred lines.