Comparative transmission genetics of introgressed chromatin in reciprocal advanced backcross populations in Gossypium (cotton) polyploids.

Introgression is a potential source of valuable genetic variation and interspecific introgression lines are important resources for plant breeders to access novel alleles. Experimental advanced-generation backcross populations contain individuals with genomic compositions similar to those resulting from natural interspecific hybridization and provide opportunities to study the nature and transmission pattern of donor chromatin in recipient genomes. Here, we analyze transmission of donor chromatin in reciprocal backcrosses between G. hirsutum and G. barbadense. Across the genome, recurrent backcrossing in both backgrounds yielded donor chromatin at slightly higher frequencies than the Mendelian expectation in BC5F1 plants, while the average frequency of donor alleles in BC5F2 segregating families was less than expected. In the two subgenomes of polyploid cotton, the rate of donor chromatin introgression was similar. Although donor chromatin was tolerated over much of the recipient genomes, 21 regions recalcitrant to donor alleles were identified. Only limited correspondence is observed between the recalcitrant regions in the two backgrounds, suggesting the effect of species background on introgression of donor segments. Genetic breakdown was progressive, with floral abscission and seed inviability ongoing during backcrossing cycles. Regions of either high or low introgression tended to be in terminal chromosomal regions that are generally rich in both genes and crossover events, with long stretches around the centromere having limited crossover activity resulting in relatively constant low introgression frequencies. Constraints on fixation and selection of donor alleles highlights the challenges of utilizing introgression breeding in crop improvement.

Identification of small effect quantitative trait loci of plant architectural, flowering, and early maturity traits in reciprocal interspecific introgression population in cotton.

Plant architecture, flowering time and maturity traits are important determinants of yield and fiber quality of cotton. Genetic dissection of loci determining these yield and quality components is complicated by numerous loci with alleles conferring small differences. Therefore, mapping populations segregating for smaller numbers and sizes of introgressed segments is expected to facilitate dissection of these complex quantitative traits. At an advanced stage in the development of reciprocal advanced backcross populations from crosses between elite Gossypium hirsutum cultivar 'Acala Maxxa' (GH) and G. barbadense 'Pima S6' (GB), we undertook mapping of plant architectural traits, flowering time and maturity. A total of 284 BC4F1 and BC4F2 progeny rows, 120 in GH and 164 in GB background, were evaluated for phenotype, with only 4 and 3 (of 7) traits showing significant differences among progenies. Genotyping by sequencing yielded 3,186 and 3,026 SNPs, respectively, that revealed a total of 27 QTLs in GH background and 22 in GB, for plant height, days to flowering, residual flowering at maturity and maturity. More than of 90% QTLs identified in both backgrounds had small effects (%PV < 10), supporting the merit of this population structure to reduce background noise and small effect QTLs. Germplasm developed in this study may serve as potential pre-breeding material to develop improved cotton cultivars.

Improved Upland Cotton Germplasm for Multiple Fiber Traits Mediated by Transferring and Pyramiding Novel Alleles From Ethyl Methanesulfonate-Generated Mutant Lines Into Elite Genotypes.

Ethyl methanesulfonate (EMS) mutagenesis offers important advantages for improving crops, such as cotton, with limited diversity in elite gene pools. EMS-induced point mutations are less frequently associated with deleterious traits than alleles from wild or exotic germplasm. From 157 mutant lines that have significantly improved fiber properties, we focused on nine mutant lines here. A total of eight populations were developed by crossing mutant lines in different combinations into GA230 (GA2004230) background. Multiple lines in each population were significantly improved for the fiber trait that distinguished the donor parent(s), demonstrating that an elite breeding line (GA230) could be improved for fiber qualities using the mutant lines. Genotypes improved for multiple fiber traits of interest suggesting that allele pyramiding is possible. Compared to midparent values, individual progeny in the population conferred fiber quality improvements of as much as 31.7% (in population O) for micronaire (MIC), 16.1% (in population P) for length, 22.4% (in population K) for strength, 4.1% (in population Q) for uniformity, 45.8% (in population N) for elongation, and 13.9% (in population O) for lint percentage (lint%). While further testing for stability of the phenotype and estimation of yield potential is necessary, mutation breeding shows promise as an approach to reduce the problem of the genetic bottleneck of upland cotton. The populations developed here may also contribute to identifying candidate genes and causal mutations for fiber quality improvement.

The Ligon lintless -2 Short Fiber Mutation Is Located within a Terminal Deletion of Chromosome 18 in Cotton.

Extreme elongation distinguishes about one-fourth of cotton (Gossypium sp.) seed epidermal cells as "lint" fibers, useful for the textile industry, from "fuzz" fibers (<5 mm). Ligon lintless-2 (Li 2 ), a dominant mutation that results in no lint fiber but normal fuzz fiber, offers insight into pathways and mechanisms that differentiate spinnable cotton from its progenitors. A genetic map developed using 1,545 F2 plants showed that marker CISP15 was 0.4 cM from Li 2 , and "dominant" simple sequence repeat (SSR) markers (i.e. with null alleles in the Li 2 genotype) SSR7 and SSR18 showed complete linkage with Li 2 Nonrandom distribution of markers with null alleles suggests that the Li 2 phenotype results from a 176- to 221-kb deletion of the terminal region of chromosome 18 that may have been masked in prior pooled-sample mapping strategies. The deletion includes 10 genes with putative roles in fiber development. Two Glycosyltransferase Family 1 genes showed striking expression differences during elongation of wild-type versus Li 2 fiber, and virus-induced silencing of these genes in the wild type induced Li 2 -like phenotypes. Further, at least 7 of the 10 putative fiber development genes in the deletion region showed higher expression in the wild type than in Li 2 mutants during fiber development stages, suggesting coordinated regulation of processes in cell wall development and cell elongation, consistent with the hypothesis that some fiber-related quantitative trait loci comprise closely spaced groups of functionally diverse but coordinately regulated genes.

Molecular Dissection of Quantitative Variation in Bermudagrass Hybrids (Cynodon dactylon x transvaalensis): Morphological Traits.

Bermudagrass (Cynodon (L.)) is the most important warm-season grass grown for forage or turf. It shows extensive variation in morphological characteristics and growth attributes, but the genetic basis of this variation is little understood. Detection and tagging of quantitative trait loci (QTL) affecting above-ground morphology with diagnostic DNA markers would provide a foundation for genetic and molecular breeding applications in bermudagrass. Here, we report early findings regarding genetic architecture of foliage (canopy height, HT), stolon (stolon internode length, ILEN and length of the longest stolon LLS), and leaf traits (leaf blade length, LLEN and leaf blade width, LW) in 110 F1 individuals derived from a cross between Cynodon dactylon (T89) and C. transvaalensis (T574). Separate and joint environment analyses were performed on trait data collected across two to five environments (locations, and/or years, or time), finding significant differences (P < 0.001) among the hybrid progeny for all traits. Analysis of marker-trait associations detected 74 QTL and 135 epistatic interactions. Composite interval mapping (CIM) and mixed-model CIM (MCIM) identified 32 main effect QTL (M-QTL) and 13 interacting QTL (int-QTL). Colocalization of QTL for plant morphology partially explained significant correlations among traits. M-QTL qILEN-3-2 (for ILEN; R2 = 11-19%), qLLS-7-1 (for LLS; R2 = 13-27%), qLEN-1-1 (for LLEN; R2 = 10-11%), and qLW-3-2 (for LW; R2 = 10-12%) were 'stable' across multiple environments, representing candidates for fine mapping and applied breeding applications. QTL correspondence between bermudagrass and divergent grass lineages suggests opportunities to accelerate progress by predictive breeding of bermudagrass.

Genetic Analysis of Gossypium Fiber Quality Traits in Reciprocal Advanced Backcross Populations.

In mapping populations segregating for many loci, the large amount of variation among genotypes often masks small-effect quantitative trait loci (QTL). This problem can be reduced by development of populations with fewer chromosome segments segregating. Here, we report early QTL detection in reciprocal advanced backcross populations from crosses between elite Gossypium hirsutum L. 'Acala Maxxa' (GH) and G. barbadense L. 'Pima S6' (GB). A total of 297 BCF and BCF progeny rows-127 segregating for GB chromosome segments in GH background and 170 segregating for GH chromosome segments in GB background-were evaluated in three environments. Totals of 3186 and 3026 polymorphic single-nucleotide polymorphisms (SNPs) in GH and GB backgrounds, respectively, were identified and used for trait mapping. Small-effect QTL (<10% variance explained) made up 87 and 100% of QTL in GH and GB backgrounds, respectively. In both species, favorable alleles were found with effects being masked or neutralized by unfavorable alleles, with greater scope for improvement of GH than GB by introgressive breeding. A total of three stable QTL-two in GH background for fiber elongation (ELO) and micronaire (MIC) and one in GB background for upper-half mean length (UHM)-were identified in two out of three environments. Curiously, only four QTL-three for UHM and one for ELO-showed the expected opposite effects in reciprocal backgrounds, perhaps reflecting the combined consequences of epistasis, small phenotypic effects, and low coverage of some genomic regions. Along with new information for marker-assisted breeding, this study adds to knowledge that can be used to unravel complex genetic networks governing fiber quality traits.

SSR-enriched genetic linkage maps of bermudagrass (Cynodon dactylon × transvaalensis), and their comparison with allied plant genomes.

KEY MESSAGE: We report SSR-enriched genetic maps of bermudagrass that: (1) reveal partial residual polysomic inheritance in the tetraploid species, and (2) provide insights into the evolution of chloridoid genomes. This study describes genetic linkage maps of two bermudagrass species, Cynodon dactylon (T89) and Cynodon transvaalensis (T574), that integrate heterologous microsatellite markers from sugarcane into frameworks built with single-dose restriction fragments (SDRFs). A maximum likelihood approach was used to construct two separate parental maps from a population of 110 F1 progeny of a cross between the two parents. The T89 map is based on 291 loci on 34 cosegregating groups (CGs), with an average marker spacing of 12.5 cM. The T574 map is based on 125 loci on 14 CGs, with an average marker spacing of 10.7 cM. Six T89 and one T574 CG(s) deviated from disomic inheritance. Furthermore, marker segregation data and linkage phase analysis revealed partial residual polysomic inheritance in T89, suggesting that common bermudagrass is undergoing diploidization following whole genome duplication (WGD). Twenty-six T89 CGs were coalesced into 9 homo(eo)logous linkage groups (LGs), while 12 T574 CGs were assembled into 9 LGs, both putatively representing the basic chromosome complement (x = 9) of the species. Eight T89 and two T574 CGs remain unassigned. The marker composition of bermudagrass ancestral chromosomes was inferred by aligning T89 and T574 homologs, and used in comparisons to sorghum and rice genome sequences based on 108 and 91 significant blast hits, respectively. Two nested chromosome fusions (NCFs) shared by two other chloridoids (i.e., zoysiagrass and finger millet) and at least three independent translocation events were evident during chromosome number reduction from 14 in the polyploid common ancestor of Poaceae to 9 in Cynodon.