2009. The genetic architecture of maize photoperiod sensitivity as defined by recombinant inbred line backcross, and heterogeneous inbred family populations.

2009. The genetic architecture of maize photoperiod sensitivity as defined by recombinant inbred line backcross, and heterogeneous inbred family populations.

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ABSTRACT

Tropical maize germplasm has frequently been cited as a potential source of enhanced genetic diversity that could be used to increase corn productivity. One obstacle to utilizing tropical maize germplasm in temperate breeding programs is photoperiod sensitivity, which is very common in tropical adapted maize lines. An investigation of the quantitative trait loci (QTL) contributing to maize photoperiod sensitivity may increase the facility with which maize breeders can adapt tropical maize germplasm to temperate latitudes.

The photoperiod sensitive phase of maize was studied in a diverse set of inbreds. From a factorial mating of two temperate and two tropical inbreds, four populations of recombinant inbred lines (RIL) were developed for the purpose of mapping the QTL underlying photoperiod sensitivity in tropical maize. Plants were grown in both long- and short-day environments and a number of traits were measured in each environment. These traits include flowering time, plant height, leaf number, and ear structure traits. The trait differences between long- and short-day environments were reported as the photoperiodic responses of the RILs. Utilizing the data of both individual and combined mapping populations, QTL were identified using iterative QTL mapping (iQTLm). The positions and effects of these QTL were compared between populations and with flowering time, plant height, and leaf number QTL from other mapping studies. We detected four regions in the genome that produced large photoperiodic effects and named these Zea mays Photoperiodic Response 1-4 (ZmPRl, ZmPR2, ZmPR3, and ZmPR4). Similar QTL positions have been detected by other researchers studying photoperiod sensitivity and flowering time in maize. In addition to a major QTL on chromosome 3, QTL affecting the tasseled ear phenotype of maize were also found in the ZmPR3 and ZmPR4 QTL regions, implicating these QTL as also having effects on floral morphology. The four ZmPR loci are the most promising targets of marker-assisted selection against photoperiod sensitivity in maize.

Verification of the four ZmPR QTL was undertaken in four BC2F3:4 mapping populations, each having B73 as the recurrent parent. The CML254 backcross population had the same parentage as one of the RIL populations originally used to identify the ZmPR loci. We verified the presence of three of the four ZmPR loci in this population. We also found that the other three mapping populations, derived from CML247, Ki3, and Kil 1 showed significant flowering time and plant height associations at some of the ZmPR loci. Winter nurseries were used to verify that the ZmPR4 QTL, which was the strongest photoperiodic flowering time QTL detected in previous mapping studies, was indeed a photoperiodic and not flowering time per se QTL. An Ft population derived from a Kil 4 x CML254 cross was used to identify functional allelic differences among these two tropical lines at the ZmPR loci. Alleles of Ki 14 and CML254 were functionally distinct at ZmPR4 and ZmPR2.

The utility of QTL mapping for applied breeding programs is often limited by the transferability of QTL across populations and by the lack of precision in QTL mapping. One solution to these obstacles is to fine-map and clone the gene or genes underlying the QTL. I developed several heterogeneous inbred families (HIFs) from four RIL populations in order to facilitate fine-mapping. I observed several traits in these HIFs in phytotron, greenhouse, and field environments. I report the manner with which the HIFs were derived, as well as some observations and notes about future fine-mapping directions with these HIFs  .