Prevalence of Linear Configuration among Chain Trivalents at Metaphase I in Pollen Mother Cells of Petunia axillaris (LAM.) B.S.P.

Abstract

At somatic metaphase the kinetochore of each sister chromatid interacts with the spindle in such a way that the two subsequently disjoin one to each pole. At metaphase I of meiosis in disomics, a pair of chromatids is one half of a bivalent. Here, in their involvement with the spindle, the two kinetochores of a chromatid pair are not poised disjunctionally, and the centromere is thus said to be syntelic. For a 'double kinetochore' of each member of a meiotic bivalent, 'syntelic' aspect is thus a derived state. Failure of the 'double kinetochore' to attain to 'syntelic' state is rare for the members of a bivalent. On the other hand, the 'kinetochore pair' of a univalent at meiosis I may assume an amphitelic state at metaphase I even if the initial development was towards a syntelic state (Bauer et al. 1961, Sybenga 1975). When we consider chain trivalents, which indeed are the simplest of multivalents, syntelic state for all three kinetochore pairs is quite prevalent and is often seen in the alternate configuration where the mid-member is poised disjunctionally from both its neighbours. Much rarer is the variant, where the three syntelic centromeres disjoin in a 2-1 manner as in the case of ad jacent orientation.In several species, the frequency of non-linear forms is so overwhelmingly great that the 'linear configuration' is not thoroughly described.A serious consideration of the linear forms is the main theoretical difference between two recent reviews (Rickards 1983, Sybenga andRickards 1987).The descriptive data are of greater interest when they come from species where a large proportion of chain trivalents assume a 'linear form'.It is possible that a 'kinetochore pair' which fails to attain syntelic status reverts to an amphitelic behaviour pattern.But it is more rigorous to describe it as 'non-syntelic', when it is as yet part of an association.Petunia pollen mother cells at metaphase I are not ideal for light microscopic studies of acetocarmine squash preparations.However, in view of the prevalence of linear forms, their description is of some interest. Material and methodsTriploid plants were raised from reciprocal crosses between diploids and colchicine induced tetraploids of Petunia axillaris (Lam.) B. S. P. For meiotic study, young flower buds of 0.2 to 0.4mm size were fixed in 1:4 acetic alcohol between 9:00 and 11:00 a.m. Smear preparations of PMC's were made in 1% acetocarmine and in alcoholic hydrochloric acid carmine (Snow 1963). Paucity of interstitial chiasmata and disjunction of at least one con figuration per PMC were taken as criteria to discriminate late metaphase I cells from mid metaphase. Quantitative data on chain trivalents at metaphase I were collected from intact cells when the entire chromosome complement could be made out. In addition, trivalent data in cells with clumped chromosomes were separately recorded.