Mapping QTLs for pollen thermotolerance in recombinant inbreds
--Carla Frova, Michela Bossolasco and Mirella Sari Gorla

Pollen thermotolerance is a major component of yield stability under high temperature stress. Here we report its genetic dissection through RFLP analysis of a recombinant inbred line population (T232 X CM37, provided by B. Burr, B, Brookhaven National Laboratory). The character was measured in vitro as the degree of injury [I = (1 - T/C) * 100] of pollen germination ability (IPGG) and of pollen tube growth (IPTG), caused by high temperature treatment (T = 41 C) in comparison with control (C = 27 C) growth conditions. Both IPGG and IPTG showed a typical quantitative distribution among RIs and high heritability: h2 = 0.64 and 0.68 respectively. Regression analysis between each RFLP locus and trait expression identified several markers significantly correlated with pollen thermotolerance. They are shown in Figure 1A and B, where each marker is represented by its R2 value. In order to avoid false QTL assignments, the correlation matrices between all significant markers for each trait were analyzed; in each group of correlated markers only the one with the highest R2 value was considered indicative of the presence of a QTL in that region. The results show that at least five genomic regions are involved in IPGG and six in IPTG determination, and that the two traits are controlled by different sets of genes. A comparison between the regions identified and those (also determined in this study) containing putative QTLs for pollen germination and tube growth in non-stress conditions, indicate that the "base" and the injury traits are largely independent. 

Figure 1. Localization of putative QTLs controlling IPGG (A) and IPTG (B) in maize. Horizontal bars indicate degree of correlation between RFLP loci and the characters in terms of R2. Significant loci are indicated by asterisks.

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