Genetic and cytological analysis of a novel type of low temperature-dependent intrasubspecific hybrid weakness in rice.

Hybrid weakness (HW) is an important postzygotic isolation which occurs in both intra- and inter-specific crosses. In this study, we described a novel low temperature-dependent intrasubspecific hybrid weakness in the F1 plants derived from the cross between two indica rice varieties Taifeng A and V1134. HW plants showed growth retardation, reduced panicle number and pale green leaves with chlorotic spots. Cytological assay showed that there were reduced cell numbers, larger intercellular spaces, thicker cell walls, and abnormal development of chloroplast and mitochondria in the mature leaves from HW F1 plants in comparison with that from both of the parental lines. Genetic analysis revealed that HW was controlled by two complementary dominant genes Hw3 from V1134 and Hw4 from Taifeng A. Hw3 was mapped in a 136 kb interval between the markers Indel1118 and Indel1117 on chromosome 11, and Hw4 was mapped in the region of about 15 cM between RM182 and RM505 on chromosome 7, respectively. RT-PCR analysis revealed that only LOC_Os11g44310, encoding a putative calmodulin-binding protein (OsCaMBP), differentially expressed among Taifeng A, V1134 and their HW F1. No recombinant was detected using the markers designed based on the sequence of LOC_Os11g44310 in the BC1F2 (Taifeng A//Taifeng A/V1134) population. Hence, LOC_Os11g44310 was probably the candidate gene of Hw3. Gene amplification suggested that LOC_Os11g44310 was present in V1134 and absent in Taifeng A. BLAST search revealed that LOC_Os11g44310 had one copy in the japonica genomic sequence of Nipponbare, and no homologous sequence in the indica reference sequence of 9311. Our results indicate that Hw3 is a novel gene for inducing hybrid weakness in rice.