The relation between synapsis and chiasma formation in Rhoeo spathacea

Abstract

Rhoeo spathacea (Schwartz) Stearn (2n=12) is a complete translocation heterozygote in which a ring of 12 chromosomes is often observed from diakinesis through metaphase I. A new hypotonie bursting technique on primary microsporocytes made possible visualization of the complete pattern of pachytene synapsis in R. spathacea by light and electron microscopy. Analysis showed that most of the length of a chromosome never synapsed and that relational twisting of the lateral elements was prominent in the distal segments of most synaptonemal complexes (SCs). The most common situation was 12 relatively short SCs representing the 24 synapsed terminal segments of the 12 chromosomes; 11 and 10 SCs were also observed but less frequently, indicating one and two synaptic failures, respectively. Similarly, in diakinesis a ring of 12 chromosomes was most common, but a chain of 12 chromosomes and two chains of chromosomes could also occur. These situations indicated zero, one, and two chiasma failures, respectively. The frequencies of synaptic failure (0.021) and of chiasma failure (0.022) were essentially the same as were the distributions of cells with different numbers of synaptic and chiasma failures. These comparisons indicate that if synapsis occurs between two distal segments of the chromosomes of R. spathacea, a chiasma will almost certainly follow. A diagram to illustrate the arrangement of mid-pachytene chromosomes and a model to explain the origin of relational twisting of lateral elements in SCs are presented.