The studies conducted in cotton showed that the majority of the SDs were mainly skewed towards the male parent rather than the female population, as was observed on chromosome step 18 (Dai et al. 2017). However, in all the studies conducted to unravel the mystery of SDs in cotton, no experiment has been undertaken to explore the SDs in the F2:3 population derived from the diploid wild cotton parental lines. hirsutum as the recurrent parent and G. mustelinum as the donor cultivar (Chandnani et al. 2017). And therefore, to explore the phenomena of the SDs in wild cotton progenitors, an interspecific population between G. klotzschianum and G. davidsonii, and between G. thurberi and G. trilobum were developed. The four parental lines were primarily selected because of their diverse genetic traits and broader ecological niches. The four parental lines used in the construction of the genetic maps are known to have traits for resistance to bacterial blight (G. davidsonii) (Zhang et al. 2016), sucking pests such as aphids (G. klotzschianum) (Wei et al. 2017), Fusarium wilt, silver leaf whitefly and cotton bollworm resistance (G. thurberi) (Natwick 2006), Verticillium wilt (G. trilobum) (Dong et al. 2019). A total of 188 individuals were genotyped using SSR markers, primarily focusing on the exploitation of the genetic mechanism of the SD in severely distorted chromosome D502 and chromosome D507. The analysis of the SD from the genetic maps constructed from the diploid cotton of the D genome was conducted. The first map was then generated from two closely related parents, G. klotzschianum and G. davidsonii (Kirungu et al. 2018) and the second map developed from G. thurberi and G. trilobum (Li et al. 2018), in either of the maps, the F2:step 3 population used, the genotypic data from the two maps were combined to generate the consensus map, and the consensus map was generated by using the two maps. The only available maps developed from the wild cotton species of the D genome. The focus was on chromosome D502 and chromosome D507 which showed severe distortions of markers from the two maps. Moreover, the marker segregation and genes within the SDRs were mined and analyzed. The genes mined within the SDR and understanding their roles will be significant in elucidating the role played by segregation distortion, and will help in improving the elite cultivated cotton germplasms with ever-shrinking genetic base and significantly lower adaptive mechanisms to various abiotic and biotic stress factors.
The two genetic maps were generated from an interspecific population obtained from the four parental lines. The first genetic map (Map A) was constructed from the F2:3 population derived El Paso hookup sites from the self-pollinating F1 population of G. klotzschianum (female parent) and G. davidsonii (male parent). Similarly, the second genetic map (Map B) was constructed from F2:step 3 populations derived from G. thurberi (female parent) and G. trilobum (male parent). A total of 188 progenies were used as the mapping population. The F2:3 progenies from the four parental lines were developed and grown in the wild cotton nurseries, managed by the Institute of Cotton Research, Chinese Academy of Agricultural Sciences (ICR, CAAS), located in Sanya, Hainan province, China. The development of the F2:3 progenies followed a similar pattern as described by Magwanga et al. (2020) in the development of the backcross progenies between G. tomentosum (donor male parental line) and G. hirsutum (recurrent female parental line).
Unit markers genotyping
Total DNA was extracted from the F2:3 progenies and their parental lines using the CTAB method (Zhang et al. 2000b). Polymerase chain reaction (PCR) was conducted. The amplified PCR products were electrophoresed on non-denaturing 10% polyacrylamide gel electrophoresis in the 1 ? TBE buffer, and the gels were then visualized after silver staining (Huang et al. 2018). The primers used were the SWU markers which were developed by Southwest University in China, hence the acronym SWU. In the construction of the genetic map A, a total of 12 560 SWU markers were screened of which 1 000 markers were found to be polymorphic. Out of the 1 000 polymorphic markers, 728 markers were mapped and generated 13 linkage groups, designated as chromosome D501 to D513. In the second genetic map, map B, 12 560 SWU markers were screened, of which 996 markers were polymorphic, and only 849 polymorphic markers were mapped onto the 13 linkage groups. For the construction of consensus map, 1 492 polymorphic markers were applied to generate the genetic map, after removing the duplicated markers. The details of the markers and their sequences are shown in Supplementary Table S1.