Genetic Analysis And Gene Mapping Of The Fuzzless And Hybrid Lethality Traits In Cotton

Cotton is an important cash crop. Its fiber is the most important natural textile material and it plays an important role in national economy of our country and all over the world. Cotton fibers used in textiles are unicellular trichomes originating from the outer epidermal layer of the ovule. Fibers are classified into two types, lint and fuzz. During the ginning process the economically important lint fibers are removed from the seed, leaving the much shorter fuzz fiber. The fuzzless seed phenotype is believed to be a beneficial trait because of the ease and cleanliness of ginning. The fuzzless seed lines are a valuable resource in the study of trichome (lint) initiation, development and cotton breeding.In this study, the fuzzless mutants (FLMs) were crossed with several wild fuzz parents, including G. hirsutum L. and G. barbadense. The genetic analysis of the fuzz trait was conducted and both Nj and n2 were tagged using the molecular markers. Given these interspecific and intraspecific mapping data, a duplicate inheritance of the recessive fuzzless trait in allotetraploid cotton is assumed. In addition, the inherence of lint percentage was studied in the cross between N1FLM and TM-1, and this will provide theoretical basis for fuzzless cotton breeding.During the research of the fuzzless trait in cotton, we discovered the interspecfic hybrid lethality between G. hirsutum and G. barbadense cv. CoastlandR4-4. Hybrid lethality is a major mechanism of isolation which plays an important role in the process of speciation. Clarifying the mechanisms of hybrid lethality is important to understand biological speciation. To elucidate the general mechanisms underlying reproductive isolation, it is necessary to identify examples of these phenomena, study the genetic basis and clone the genes involved. In addition, the causal genes could be employed or removed as necessary for crop breeding. In this study, we discovered a novel lethal combination in interspecific hybrids between G. hirsutum and G. barbadense cv. CoastlandR4-4. To elucidate the molecular mechanism underlying hybrid lethality and manipulate the causal genes, we conducted genetic analysis and map the causative genes using the detailed linkage map (Guo et al.2007).1. Genetic analysis of the fuzzless phenotype in cottonThe fuzzless mutants (FLMs) were crossed with wild fuzz parents, including several cultivars of G. barbadense and genetic standard line of G. hirsutum, TM-1.F1 seeds produced self-pollinated F2 progeny and BC1 was produced from some crosses. Fuzz scoring was conducted in the parents, F1, F2 and BC1 progeny. All of these data indicated that inheritance of both N1 and n2 FLMs was best defined with a one-gene model.2. Molecular mapping of the recessive fuzzless gene n2The interspecfic population (n2FLM×XH7) F2 consisted of 141 individuals was used to map n2. According to the results reported previously, the SSR markers anchored on Chr.D12 were used to detect the 141 individual plants of the interspecific F2. Linkage analysis, however, failed to show that any marker on Chr. D12 was linked to this recessive fuzzless gene. The SSR markers anchored on A12 were used to screen the population (n2FLM×XH7) F2. Linkage analysis showed that the recessive fuzzless locus was mapped on A12. and the nearest marker to n2 was BNL1679, at a distance of 2.7 cM.The 74 completely fuzzless individuals as the parent n2 FLM in intraspecific (n2FLM×TM-1) F2 were selected to map n2. All markers anchored on A12 and D12 (Guo et al.2007) were used to screen the two parents, n2FLM and TM-1. The polymorphic markers were used for a linkage analysis screen of the 74 extremely fuzzless plants. The pairwise recombination frequency between SSR markers, and between SSR markers with n2, was estimated using the maximum likelihood method. Recombination frequency was converted to genetic distance using the Kosambi mapping function. The linkage map was constructed with JoinMap 3.0 (LOD=28.0). The results showed that the recessive fuzzless locus was mapped on D12.3. Molecular mapping of the dominant fuzzless gene N1The population (N1FLM×XH7) F2 consisted of 141 individuals was used to map N1,. The SSR markers on A12 were used to screen the 141 F2 individuals. JoinMap 3.0 was used to construct linkage map. Linkage analysis showed that N1 was mapped on A12, and the nearest marker to it was BNL1679. at a distance of 1.9 cM.4. Genetic analysis of lint percentageN1FLM and TM-1 vary significantly in lint percentage. Analysis of the frequent distribution for lint percentage in (N1FLM×TM-1)F2 population and the lint percentage of P1、P2 and F1 indicated that lint percentage was mainly controlled by one dominant major gene derived from TM-1. Correlation analysis suggested the dominant fuzzless gene N1 had a relatively significant negative effect on lint percentage The allele at N1 decreased the lint percent, and can explain the phenotypic variation by 24.44%。The method of major gene plus polygene mixed inheritance model was used to analyze the genetics of lint percentage in the crosses N1FLM×TM-1. The results of joint analysis of multiple generations of P1、P2 F1 and F2 showed the optimum inheritance model in lint percentage was one major gene plus polygene mixed inheritance model. The heritability of major gene and polygene was 82.64% and 5.67%, respectively.5. Discovery and genetic analysis of interspecific hybrid lethalityDuring the research of the fuzzless trait in cotton, we discovered the hybrid lethality between G. hirsutum and G. barbadense cv. CoastlandR4-4. For analysis of the genetic mechanism producing the lethal phenotype, three inbred lines in G. hirsutum, N1FLM, n2FLM and TM-1, were crossed with the G. barbadense cv. CoastlandR4-4 and their resulting F1, F2 and BC1 progenies were produced. The reciprocal F1 hybrids of CoastlandR4-4×N1FLM and CoastlandR4-4×TM-1 were developed. The statistical analysis of phenotype showed that the hybrid lethality was controlled by two dominant complementary genes:one gene derived from the G. hirsutum and another gene from the G. barbadense CoastlandR4-4. This is consistent with the DM model where deleterious epistatic interactions are the primary genetic basis of hybrid lethality. In addition, in this study the different degree of lethal symptom within one population was probably caused by the dosage of causal alleles.6. Molecular mapping of the hybrid lethality genesThe populations derived from the backcross (N1/FLM×CoastlandR4-4)CoastlandR4-4 comprising of 349 individuals and (N1FLM×CoastlandR4-4)N1FLM comprising of 343 individuals were used to map the hybrid lethality genes derived from G. hirsutum and G. barbadense CoastlandR4-4. respectively. In order to find quickly on which chromosome the genes might lie, the detailed linkage map (Guo et al.,2007) and the BSA method (Michelmore et al.,1991) were used. We found several SSR markers on Chr. D8 that linked to the hybrid lethality gene from G. hirsutum. The SSR markers on Chr. D8 were further used to detect the individuals of (N1FLM×CoastlandR4-4) CoastlandR4-4. Joinmap3.0 was used to construct the genetic linkage map. The resulting illustrated the causal gene from G. hirsutum located in Chr. D8 and the closest marker was NAU2306, at a distance of 3.4 cM. In the same way, the causal gene from G. barbadense CoastlandR4-4 was mapped on Chr. D11. The nearest marker to it was BNL1154, at a distance of 4.1 cM. In view of the different symptoms or the different locations of the hybrid lethality genes from that of previous reports, our results indicated that they were novel Dobzhansky-Muller genes for the induction of hybrid lethality in cotton. According to the gene nomenclature, the genes from the G. hirsutum and from the G. barbadense CoastlandR4-4 were named as Le3 and Le4, respectively.