CYTOGENETIC STUDY OF DIPLOID ACTINIDIA CHINENSIS - KARYOTYPE, MORPHOLOGY OF SEX CHROMOSOMES AT PRIMARY DIFFERENTIATION STAGE AND EVOLUTIONARY SIGNIFICANCE

Abstract

The karyotype of diploid Actinidia chinensis is 2n = 2 = 58 = 38m + 18sm (2SAT) + 2t. The ratio of the largest chromosome to the shortest chromosome is 2.35 and the karyotype is type 2B. Although the two SAT-chromosomes have similar lengths and shapes, they are probably the chromosomes reponsible for sex determination. The SAT region of nucleolar chromosomes may be the region of sex determination. At the pachytene stage of pollen mother cell (PMC) meiosis, the two-nucleolar chromosomes of pistillate plants pair tightly, in a linear manner. In staminate plants the SAT region is about 15% of the total nucleolar chromosomal length and the nucleolar chromosomes are obviously heteromorphic and they cannot pair; for the remaining 85% of their length (i.e., non-SAT regions, chromosomes can pair tightly, making the nucleolar divalent of staminate plants display a “Y” shape. Therefore, it is assumed that sex chromosomes are probably at an early stage of differentiation and that the sex determination mechanism is probably of the XX/XY type. Because of the configuration character of PMC chromosomes during M1 (1-2 quadruples or 1-2 secondary pairings between divalents) and because the 8th and 9th pairs and 11th and 12th pairs of chromosomes respectively are similar in length, Lt/St and in morphology, it is concluded that diploid Actinidia chinensis is probably derived from a paleotetraploid. The comparison of cytological characters between Actinidia ( =29) and related genera Clematoclethra =24, Saurauia ; =15）shows that Actinidia is a relatively evolved genus. Morphological analysis of the chromosomes at the pachytene stage may be an effective approach to a study of the sex determination mechanisms of dioecious plants, which are at an early stage of differentiation.