Chromosome number reduction in the sister clade of Carica papaya with concomitant genome size doubling.

PREMISE OF THE STUDY Caricaceae include six genera and 34 species, among them papaya, a model species in plant sex chromosome research. The family was held to have a conserved karyotype with 2n = 18 chromosomes, an assumption based on few counts. We examined the karyotypes and genome size of species from all genera to test for possible cytogenetic variation. METHODS We used fluorescent in situ hybridization using standard telomere, 5S, and 45S rDNA probes. New and published data were combined with a phylogeny, molecular clock dating, and C values (available for ∼50% of the species) to reconstruct genome evolution. KEY RESULTS The African genus Cylicomorpha, which is sister to the remaining Caricaceae (all neotropical), has 2n = 18, as do the species in two other genera. A Mexican clade of five species that includes papaya, however, has 2n = 18 (papaya), 2n = 16 (Horovitzia cnidoscoloides), and 2n = 14 (Jarilla caudata and J. heterophylla; third Jarilla not counted), with the phylogeny indicating that the dysploidy events occurred ∼16.6 and ∼5.5 million years ago and that Jarilla underwent genome size doubling (∼450 to 830-920 Mbp/haploid genome). Pericentromeric interstitial telomere repeats occur in both Jarilla adjacent to 5S rDNA sites, and the variability of 5S rDNA sites across all genera is high. CONCLUSIONS On the basis of outgroup comparison, 2n = 18 is the ancestral number, and repeated chromosomal fusions with simultaneous genome size increase as a result of repetitive elements accumulating near centromeres characterize the papaya clade. These results have implications for ongoing genome assemblies in Caricaceae.