Graduate Thesis Or Dissertation
 

Localization and genetic mapping of some factors influencing rachis brittleness and free-threshing habit in wheat

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  • Domesticated forms of wheat exhibit traits that have increased their adaptation to cultivation by humans. Some of the most critical adaptive differences involve changes to morphological features that make the crop easier to harvest like ear rachis stiffness (brittle to non-brittle rachis) and the ease with which the seed is released from its enclosing leaf-like structures (non-free-threshing to free-threshing). The brittle rachis trait is primarily controlled by genes on homoeologous group 3 chromosomes (3A, 3B and 3D) while the free-threshing phenotype is controlled by genes on group 2 chromosomes (2A, 2B, and 2D) in tetraploid and hexaploid wheat (Triticum turgidum L. and T aestivum L., respectively). In order to broaden our understanding of the genetic basis of these domestication traits, this research was undertaken to more precisely localize some of these factors. Two populations of recombinant inbred chromosome lines for chromosome 3A and chromosome 3B (RICL-3A and RICL-3B) were used to localize brittle rachis 2 (Br-A 2) and brittle rachis 3 (Br-A 3), two major foci that control the brittle rachis character. Using the RICL-3A population, Br-A2 was localized to a 10.9-cM region between Xgwm2 and Xbarcl9 on chromosome 3A. Another factor in the RICL-3B population, Br-A 3 was localized to a 44.9-cM region between Xbarc2I8 and Xwmc54O on chromosome 3B. With respect to the free-threshing habit, a recombinant inbred line (RIL) population developed by the International Triticeae Mapping Initiative (ITMI) and F₂ progeny (CS/CS2D F₂) of a cross between Chinese Spring and a 2D2 substitution line [Chinese Spring (tauschii 2D)] were used. Quantitative trait mapping revealed that two QTL on chromosome 2D affected both threshability and glume tenacity in the ITMI population. The locus underlying one QTL was tenacious glumes I (Tgl) that was localized to a 23-cM region flanked by Xwmc25 and XgdmlO7. The other QTL was localized near Xgwm455 and the factor responsible for it, designated tenacious glumes 3 (Tg3) was subsequently localized to an 11.3-cM interval between XbcdIO2 and Xgwm455. Two QTL also affected glume tenacity in the CS/CS2D F₂ population. One QTL corresponded to the QTL identified in the ITMI population. This QTL also represented the action of Tgl and was localized to a 20.8-cM interval between the markers Xwmc5O3 and Xbarcl68. The other QTL designated Q.Gt.orst-2D.3 was near Xgwml57. The identities of Br-A2, Br-A3, TgI, Tg3, and Q.Gt.orst-2D.3 are not known but their localization on linkage maps represents a first step towards their eventual isolation and characterization.
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