The genetic mechanism of sex type, a complex quantitative trait, in Ricinus communis L.

Abstract

Castor (Ricinus communis L.) is an important industrial crop. Castor oil is very important to the global special chemical industry as feedstock and biofuels. The global demand for castor oil is rising rapidly, however, the low-yield varieties has been the bottleneck of castor industry. Breeding high-yield hybrids using pistillate line has been the main approach of castor breeding, but it progressed slowly due to the limited knowledge to the genetic mechanism of pistillate character (PC). In this study, firstly, the experiments for male fertility restoration of 2 pistillate lines, YC1 and YC2, were conducted with 98 restoring lines. Secondly, the genetic analysis of these two pistillate lines were performed with 6 segregating populations derived from them. Thirdly, the QTLs conferring the PC were mapped with 3 different F2 populations under 2 environments with SSR markers, demonstrating that the PC was a complex quantitative trait controlled by major genes and polygenes. Totally, 16 QTLs were identified in 3 populations with a phenotypic variation explained (PVE) of 74.51%, 41.71% and 48.98% respectively, one of which was simultaneously detected in three populations and another in two populations. In addition, it was found that the epistasis effect was the important genetic component of PC, with a PVE of 29.78% and 26.24% in 2 populations respectively. Forthly, 5 QTLs underlying interspersed staminate flowers (ISF) were detected, proving that it was one of the sex types controlled by the same set of genes in castor. Conclusively, the PC was controlled by a number of recessive genes with both accumulative effect and overlapping effect, at least one pair of dominant epistatic genes and polygenes, that is, enough accumulated recessive genes was necessary for PC to express, if over, it still performed pistillate but extra more restoring genes would be needed to recover the reduntant pistillate genes. The presence of dominant epistatic genes were also necessary for the recessive genes to express the PC, otherwise the ISF would be possiple to happen. The ISF genes were also modified by polygene and the relative strength between the pistillate genes and the ISF genes determined the ISF expression. The conclusion revealed the genetic mechanism of castor sex type, providing not only a guidance for castor heterosis utilization but also a reference for genetic research of sex type in other unisexual flower plants.