CHROMOSOME-NUMBER AND SECONDARY CONSTRICTION VARIATION IN 51 ACCESSIONS OF A CITRUS GERMPLASM BANK

The mitotic chromosomes of 51 citrus accessions from the Centro Nacion al de Pesquisa em Mandioca e Fruticultura Tropical, Cruz das Almas, BA , Brazil, were analyzed. The sample included representatives of 20 Cit rus species, one of Poncirus and seven hybrids. All accessions showed 2n = 18 without any evidence of numerical variation. The most clearly variable karyotype feature was the number and position of secondary co nstrictions (SECs). In 19 accessions the SECs were not identified, mai nly due to the degree of chromatin condensation. In the remainder they varied in number from one to three per karyotype. They were found in the proximal region of one oi the three largest chromosome pairs, in t he terminal/subterminal region of a smaller chromosome or, more seldom : terminally in a larger chromosome. Only in a few cases were such con strictions observed simultaneously in both homologues of the same chro mosome pair. The high variability of this karyotype feature may be due to the activation of this region in the previous interphase but may a lso indicate a high structural variability and heterozygosity of citru s germplasms.