Cytological heterozygosity and the hybrid origin of sweet orange [Citrus sinensis (L.) Osbeck]

Citrus sinensis chromosomes, although small in size, present a remarkable d ifferentiation of bands with the fluorochromes CMA and DAPI. These bands su ggest that some heteromorphisms are fixed in this species. To investigate t he extension of these heteromorphisms, ten cultivars of C. sinenesis were a nalysed with CMA/DAPI staining and, in some of them, the 18S-5.8S-25S rRNA and 5S rRNA genes were located by in situ hybridization. CMA/DAPI staining showed exactly the same CMA(+)/DAPI(-) banding pattern for all cultivars. I n situ hybridization revealed three 18S-5.8S-25S rRNA gene sites, two proxi mally located on two similar chromosomes and one terminally located on a th ird non-related chromosome. Two 5S rRNA gene sites were observed in this sp ecies, with one located proximal to the telomeric 18S-5.8S-25S rDNA site. B oth cytological approaches revealed an invariable, heterozygotic karyotype among sweet orange cultivars. Based on these data, the putative hybrid orig in of the species is discussed.