Mapping the jp (jumbo pollen) gene and QTLs involved in multinucleate microspore formation in diploid alfalfa

The objective of this research was to map the jumbo-pollen trait in diploid alfalfa. Homozygous recessive (jpjp) plants are characterized by the compl ete failure of post-meiotic cytokinesis during microsporogenesis resulting in 100% 4n-pollen formation. Three F-1 segregating populations were produce d and analyzed for pollen-grain production and the segregation of RFLP mark ers. The first cross combination did not segregate for the jumbo-pollen tra it, but showed a clear segregation for multinucleate (bi-, tri- and tetra-n ucleate)-microspore formation. Cytological analysis showed that few plants produced 100% normal (uninucleate) microspores, whereas most of them produc ed multinucleate microspores at a variable frequency (0-75%). Plants with m ultinucleate microspores always showed a prevalence of binucleated microspo res, even though some plants showed a background presence of tri- and tetra -nucleate microspores. QTL analysis based on ANOVA I and Stepwise Multiple Regression identified three QTLs with a highly significant effect on multin ucleate-microspore formation. Two cross combinations, subsequently executed , showed Mendelian segregation for the jumbo-pollen trait and were effectiv e in locating the jp, gene on linkage group 6 close to the VgIG1b RFLP locu s, Interestingly, this RFLP locus was also linked to one QTL for multinucle ate-microspore formation. Genetic models are discussed concerning the prese nce in linkage group 6 of a cluster of genes involved in multinucleate-micr ospore formation together with possible relationships between the jp gene a nd the VglG1b QTL.