Sex Determination in Dioscorea dumetorum: Evidence of Heteromorphic Sex Chromosomes and Sex-Linked NORs.

Yams (Dioscorea spp.) are a pantropical genus located worldwide that constitute an important source of nutrients and pharmaceutical substances. Some Dioscorea crop species are widely grown in West Africa. One species that is mainly cultivated in Cameroon is Dioscorea dumetorum. This is a dioecious root crop whose sex-determining system was unknown until now. To address the possible presence of sex chromosomes in D. dumetorum, we performed a karyotype characterization of male and female individuals using classical and molecular cytogenetic approaches. It was determined that 2n = 40 was the most common number of chromosomes in all of the investigated samples. One chromosome pair was longer than the others in the chromosome set and was a heteromorph in male and homomorph in female individuals. This pair corresponded to sex chromosomes, and we also confirmed this with molecular cytogenetic experiments. The results of chromomycin banding revealed the presence of strong positive signals on this chromosome pair. The signals, corresponding to GC-rich DNA regions, were similar in size on the chromosomes of the female individuals, whereas they were different in size in the male individuals. This size difference in the GC-rich heterochromatin regions was also apparent in the interphase nuclei as one small and one large fluorescent spot. The results of the in situ hybridization experiment showed that these chromomycin positive signals on the sex chromosomes also corresponded to the 35S rDNA cluster. The mean 2C DNA value (genome size) obtained for D. dumentorum was 0.71 pg (±0.012), which represents a small genome size. We found no difference in the genome size between the male and female individuals. The results of this study contribute to increasing our knowledge of sex determination in D. dumetorum (standard sex-determining XX/XY system) and may have some agronomic applications.