Variation in sex expression of Populus, and comparative mapping analysis of sex determination across poplar species and hybrids.

The genetic mechanism of sex determination in poplar is still in its infancy. The genus Populus is generally considered as dioecious, yet, bisexual trees have been widely reported. Hence, it constitutes a working model for sex determination in flowering plants. A first evolutionary explanation of dioecious plants (Darwin, 1877), suggested that it may be advantageous to specialize on one sex rather than two as allocating resources only on one function may provide selective advantages than are comparable hermaphrodites. Recently, another argument suggested that dioecy evolved chiefly to minimize self-fertilization to avoid the deleterious effect of inbreeding depression. Moreover, a body of evidence shows that the creation of the first male or female and nascent sex chromosomes were the result of suppression of genes for female or male function in the monoecious ancestors. The genus Populus can shed light on such evolutionary explanations by providing a better understanding of the genetic and molecular mechanisms that control sex determination in dioecious plants. Here, we have embarked in an integrated genomics approach to deciphering the genetic basis of sex determination in Populus. Although, genetic mapping studies of chromosomal regions that determine sex development showed the promise of this approach, data on pedigrees are increasingly needed. A P. x canescens pedigree obtained in 1998 was carefully monitored for sex phenotype for the last 6 years. This cross revealed that bisexuality in poplar seems to proceed during time and showed segregating ratio of males, females and a significant number of bisexual trees. This allows carrying out repetitive studies to monitor changes over time. Interestingly, the male parent may probably confer the bisexual tree trait. The genetic analysis of sex locus in the P. x canescens pedigree will help unravel the discrepancy in the location of the sex locus on linkage group XIX as well as the inconsistencies in the heterogametic sex found in several other species of Populus. In addition, gene expression and transgenic studies of floral homeotic genes in Populus species provide remarkable insight on the role of these genes in sex determination. These studies of newly evolving sex-determining chromosomes and genes in Populus raise many new questions to be answered by further work.