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 2 n = 18 chromosomes, an assumption based on few counts. We examined the karyo types 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 aphylogeny, 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 2 n = 18, as do the species in two other genera. A Mexican clade of five species that includes papaya, however, has 2n = 18 (papaya), 2 n = 16 (Horovitzia cnidoscoloides), and 2 n = 14 (Jarilla caudataand 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 adjacentto 5S rDNA sites, and the variability of 5S rDNA sites across all genera is high. CONCLUSIONS: On the basis of outgroup comparison, 2 n = 18 is the ancestral number, and repeated chromosomal fusions with simultaneous genome sizeincrease as a result of repetitive elements accumulating near centromeres characterize the papaya clade. These results have implications for ongoing genome assemblies in Caricaceae.