Academic journal article Genetics

Maize Adaptation to Temperate Climate: Relationship between Population Structure and Polymorphism in the Dwarf8 Gene

Academic journal article Genetics

Maize Adaptation to Temperate Climate: Relationship between Population Structure and Polymorphism in the Dwarf8 Gene

Article excerpt

ABSTRACT

To investigate the genetic basis of maize adaptation to temperate climate, collections of 375 inbred lines and 275 landraces, representative of American and European diversity, were evaluated for flowering time under short- and long-day conditions. The inbred line collection was genotyped for 55 genomewide simple sequence repeat (SSR) markers. Comparison of inbred line population structure with that of landraces, as determined with 24 SSR loci, underlined strong effects of both historical and modern selection on population structure and a clear relationship with geographical origins. The late tropical groups and the early "Northern Flint" group from the northern United States and northern Europe exhibited different flowering times. Both collections were genotyped for a 6-bp insertion/deletion in the Dwarf8 (D8idp) gene, previously reported to be potentially involved in flowering time variation in a 102 American inbred panel. Among-group D8idp differentiation was much higher than that for any SSR marker, suggesting diversifying selection. Correcting for population structure, D8idp was associated with flowering time under long-day conditions, the deletion allele showing an average earlier flowering of 29 degree days for inbreds and 145 degree days for landraces. Additionally, the deletion allele occurred at a high frequency (>80%) in Northern Flint while being almost absent (<5%) in tropical materials. Altogether, these results indicate that Dwarf8 could be involved in maize climatic adaptation through diversifying selection for flowering time.

MAIZE arose from a single domestication that occurred in southwestern Mexico ^90OO years ago from a strain of teosinte Zea mays ssp. paruiglumis (MATSUOKA et al. 2002b). Native Americans spread maize over North and South America, generating a wide diversity of landraces adapted to local environmental conditions, from tropical to cool temperate (nowadays Canada and southern Chile). One of the prominent maize types in this adaptation history is the Northern Flint race (BROWN and ANDERSON 1947) cultivated in northeastern America during pre-Colombian times. This early flowering type has adapted to cold temperate regions and was reported to have an exceptional genetic divergence compared to other preColombian maize tropical or subtropical landraces that were cultivated in present-day Mexico, the southwestern United States, and the Caribbean islands (UOEBLEY et al. 1986). In North America, primary genetic pools (Northern Flint, tropical, and subtropical) cultivated by Native Americans were then used by colonists to create new landrace varieties. Both the historical records and genetic studies show that the subtropical Southern Dent type was crossed with Northern Flint ^2OO years ago, leading to the Corn Belt Dent type adapted to the temperate midwestern United States region (UOEBLEY et al. 1988).

With respect to maize adaptation in Europe, the work of REBOURG et al. (2003) based on both molecular and historical data revealed that at least two introductions occurred in the old continent: first, Colombus brought Caribbean material to southern Spain in 1493, and then Northern Flint material was introduced by either Spanish or French explorers from the eastern coast of North America during the first half of the sixteenth century. This second material was already cultivated on a significant scale in northern Europe by 1539 (REBOURG et al 2003; DUBREUIL et al. 2006). On the basis of molecular marker records, these studies also showed that landraces cultivated at intermediate latitudes in Europe resulted from the hybridization of these two parental types. Northern Flint material therefore has played a unique role in the adaptation of maize to temperate climates (i) in North America, southern Chile, and northern Europe as well as (ii) in independent hybridization processes in the Corn Belt and in Europe. These traditional Corn Belt Dent and European Flint landraces then played a key role in the development of hybrid breeding for the United States (DuviCK et al. …

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