Interspecific Chromosome Substitution Lines in the Genetic Improvement of Upland Cotton.

Competitiveness of US Upland (Gossypium hirsutum L.) cottons requires that new alleles useful for agronomic, fiber quality, and pest resistance must be introduced and bred into germplasm useful to plant breeders.  Thus, two key challenges in contemporary genetic improvement of cotton are 1) to better utilize germplasm resources, and 2) to gain useful information about genes that control important traits.  The tetraploid species G. barbadense, G. tomentosum, and G. mustelinum are primary germplasm resources for new genes and prospective enhancements of pest and disease resistance, agronomic performance, yield and fiber traits.  Conventional methods of interspecific introgression into cotton typically entail inbreeding immediately after hybridization or after a few backcrosses.  A complementary if not superior breeding strategy may be to develop disomic chromosome substitution (CS) lines.  Development of each CS line requires  [1] derivation of the recurrent parent, a monosomic or monotelodisomic hypoaneuploid Upland  (‘TM-1’-like) plant,  [2] hybridizing it to a donor species and backcrossing to  create a mono(telodi)somic substitution stock, [3] inbreeding to recover  a euploid disomic substitution line, and [4] confirmation of the cytogenetic and genetic constitution of the disomic lines by cytological analysis and chromosome-specific molecular markers.  We have developed 20 backcrossed (BC₅) G. barbadense chromosome or chromosome arm substitution lines (CS-B) of Upland cotton.  These CS-B lines provided important information about the effects of specific G. barbadense substituted chromosome on important fiber and agronomic traits. An additive dominance genetic model has demonstrated that they can be used effectively to improve Upland cultivar.  Moreover, we are using the CS-B lines for detection of inter-chromosomal epistasis and high-resolution definition and mapping of QTLs.  CS lines for G. tomentosum and G. mustelinum, CS-T and CS- M lines, are now also in development, and should contribute significantly to interspecific introgression for genetic analysis and Upland cotton improvement.