Chromosome number and secondary constriction variation in 51 accessions of a citrus germplasm bank

the mitotic chromosomes of 51 citrus accessions from the centro nacional de pesquisa em mandioca e fruticultura tropical, cruz das almas, ba, brazil, were analyzed. the sample included representatives of 20 citrus 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 constrictions (secs). in 19 accessions the secs were not identified, mainly 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 of the three largest chromosome pairs, in the terminal/subterminal region of a smaller chromosome or, more seldom, terminally in a larger chromosome. only in a few cases were such constrictions observed simultaneously in both homologues of the same chromosome pair. the high variability of this karyotype feature may be due to the activation of this region in the previous interphase but may also indicate a high structural variability and heterozygosity of citrus germplasms