Polyploid formation created unique avenues for response to selection in Gossypium (cotton)

Abstract

A detailed restriction fragment length polymorphism map was used to determine the chromosomal locations and subgenomic distributions of quantitative trait loci (QTLs) segregating in a cross between cultivars of allotetraploid (AADD) Gossypium hirsutum ("Upland" cotton) and Gossypium barbadense ("Sea Island," "Pima," or "Egyptian" cotton) that differ markedly in the quality and quantity of seed epidermal fibers. Most QTLs influencing fiber quality and yield are located on the "D" subgenome, derived from an ancestor that does not produce spinnable fibers. D subgenome QTLs may partly account for the fact that domestication and breeding of tetraploid cottons has resulted in fiber yield and quality levels superior to those achieved by parallel improvement of "A" genome diploid cottons. The merger of two genomes with different evolutionary histories in a common nucleus appears to offer unique avenues for phenotypic response to selection. This may partly compensate for reduction in quantitative variation associated with polyploid formation and be one basis for the prominence of polyploids among extant angiosperms. These findings impel molecular dissection of the roles of divergent subgenomes in quantitative inheritance in many other polyploids and further exploration of both "synthetic" polyploids and exotic diploid genotypes for agriculturally useful variation.

Figure 1

Likelihood intervals for QTLs mapped. Bars and whiskers indicate 1 lod (10-fold) and 2 lod (100-fold) likelihood intervals. STR, fiber strength; ELONG, fiber elongation; Dn, fiber thickness; Sept12, earliness; SFCw, fiber length; LnCV, CV in mean length of fiber by number; LB, ratio of log(locule number) to log(boll number); LogSDCT, mass of seed cotton. Solid lines connecting probes on different linkage groups indicate homoeologous chromosomal segments supported by three or more pairs of adjacent duplicated loci (15). Dotted lines connecting LGD01, chr4, and LGA05; chr1, chr15, and LGD02; and chr17b, LGD05, and chr22 indicate cases where orthology (homoeology) cannot be distinguished from paralogy (multiple duplication events: see ref. 15). Arrows indicate the inferred locations of markers used to align the homoeologous linkage groups, based on a published map (15). An asterisk denotes loci not on the published map (15). For any duplicated loci that are not consistent with inferred relationships among chromosomes (linkage groups), the homoeologous (or paralogous) loci are indicated in parentheses. Chromosomes/linkage groups that neither contain QTLs nor are homoeologous to regions containing QTLs are not shown.