Pleiotropic Effects of the Duplicate Maize FLORICAULA/LEAFY Genes Zfl1 and Zfl2 on Traits under Selection during Maize Domestication

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ABSTRACT

Phenotypic variation on which selection can act during evolution may be caused by variation in activity level of developmental regulatory genes. In many cases, however, such genes affect multiple traits. This situation can lead to co-evolution of traits, or evolutionary constraint if some pleiotropic effects are detrimental. Here, we present an analysis of quantitative traits associated with gene copy number of two important maize regulatory genes, the duplicate FLORICAULA/LEAFY orthologs zfl1 and zfl2. We found statistically significant associations between several quantitative traits and copy number of both zfl genes in several maize genetic backgrounds. Despite overlap in traits associated with these duplicate genes, zfl1 showed stronger associations with flowering time, while zfl2 associated more strongly with branching and inflorescence structure traits, suggesting some divergence of function. Since zfl2 associates with quantitative variation for ear rank and also maps near a quantitative trait locus (QTL) on chromosome 2 controlling ear rank differences between maize and teosinte, we tested whether zfl2 might have been involved in the evolution of this trait using a QTL complementation test. The results suggest that zfl2 activity is important for the QTL effect, supporting zfl2 as a candidate gene for a role in morphological evolution of maize.

THE identification of genes involved in morphological evolution in both plants and animals has benefited greatly from quantitative trait locus (QTL) studies. Genes underlying QTL causing trait differences in various plant and animal species have been identified, providing an opportunity to study the mechanisms of morphological change under selection. Two important questions for better understanding the mechanisms of the evolution of development are (1) What role do developmental regulatory genes play in the control of quantitative traits?, and (2) Do these genes underlie QTL?

Several genes responsible for QTL play important roles in normal development (MACKAY 2001). Interestingly, some of the genes identified as QTL have complex or pleiotropic effects on development. For example, the teosinte branchedl (tbl) gene, underlying a QTL involved in maize domestication, affects inflorescence sex, ear size, branch length and tiller number (DOEBLEY et al. 1997), while in tomato, the fw2.2 locus, which controls changes in fruit shape and size during domestication (GONG et al 2002; LIU et al 2003), also plays a quantitative role in fruit number and photosynthate distribution (NESBITT and TANKSLEY 2001). Identifying pleiotropic quantitative effects of regulatory genes putatively involved in evolution will help us understand the possibilities and limitations for morphological evolution in populations by shedding light on how trait correlations may arise.

In this study, we examine phenotypic variation of several traits associated with maize domestication for association with varying wild-type copy number of the paralogous maize regulatory genes zfll and zfl2 (BOMBLIES et al. 2003). These genes are orthologous to the FLOUICAULA(FLO)/ LEAFY(LFY) genes of Antirrhinum and Arabidopsis (COEN et al. 1990; WEIGEL et al. 1992). As with FLO and LFY, zfll and zfl2 control essential aspects of reproductive development, such as flower identity and patterning, that are required for fertility (BOMBLIES et al. 2003). On the basis of a preliminary finding that mutations at the zfl2 locus are associated with quantitative effects on inflorescence phyllotaxy, we proposed zfl2 as a candidate locus for a major-effect QTL for inflorescence phyllotaxy differences between maize (Zea mays ssp. mays) and its wild ancestor, teosinte (Z. mays ssp. parviglumis; BOMBLIES et al. 2003), that maps to the same region of maize chromosome 2 (DOEBLEY and STEC 1991; DOEBLEY and STEC 1993).

This study addresses the following questions:

1. What are the pleiotropic quantitative effects associated with zfll and zfl2 gene dosage in different maize genetic backgrounds? …