Cytogenetics and Reproductive Biology of Bixa orellana L. (Bixaceae)

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Cytogenetic and reproductive assays were carried out on a population of annatto (Bixa orellana) of Southeastern Brazil. The reproductive data indicate that B. orellana is self-compatible but preferentially alogamous and entomophylous. No apomictic fruits were obtained from emasculated flowers. Fruits formed exclusively by self-pollination were smaller and had fewer seeds than those obtained by open pollination, highlighting the importance of pollination for annatto seed production. The meiotic process was analyzed and some abnormalities observed resulted in only some sterile pollen (1.9%) with no significant reduction of fertility. The cytogenetic studies showed the haploid chromosome number n=7 and the diploid number 2n=14, with a remarkable bimodal karyotype. FISH with 45S rDNA and 5S rDNA probes showed that these loci do not occur in synteny. Both probes hybridized in the proximal regions of the short arm of pair 1 (45S) and pairs 4 and 6 (5S). One pair of 5S rDNA sites presented larger signals in FISH than the other pair. These karyological features suggest that fusion of chromosomes and amplification of DNA segments played an important role in karyotype evolution of B. orellana.