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

• Published in: American journal of botany Vol. 103; no. 6; pp. 1082 - 1088
• Main Authors: Rockinger, Alexander, Sousa, Aretuza, Carvalho, Fernanda A., Renner, Susanne S.
• Format: Journal Article
• Language: English
• Published: United States Botanical Society of America, Inc 01-06-2016; Botanical Society of America
• Subjects: Botany; Brassicales; Carica - genetics; Carica papaya; Caricaceae; Caudata; Chromosomes; Chromosomes, Plant - genetics; descending dysploidy; DNA, Ribosomal - genetics; Evolution, Molecular; FISH; Flowers & plants; Genome Size; genome size doubling; Genome, Plant; Genomes; Hybridization; In Situ Hybridization, Fluorescence; interstitial telomere repeats; Phylogenetics; Phylogeny; Repetitive Sequences, Nucleic Acid - genetics; Telomere - genetics; Africa; FISH; descending dysploidy; Caricaceae; interstitial telomere repeats; genome size doubling; Brassicales
• ISSN: 0002-9122; 1537-2197
• DOI: 10.3732/ajb.1600134

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 caudato 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.