Peculiarities in PMC Meiosis of Pisum sativum.

The general outline of meiotic prophase proposed by Wilson in 1925 included three stages which usually are missing in the description of meiosis today: (1) synizesis (= the synaptic stage in plants), (2) the diffuse stage, which follows the pachytene and (3) the second contraction, following the diffuse stage. The last two stages were regarded by Wilson as being very variable and could even be lacking. These stages, described by early plant cytologists later became regarded as fixation artefacts and after the introduction of the squash technique were either misinterpreted and/or forgotten.However, all three stages are easily recognised in aceticethanol fixed preparations of plant meiosis and are demonstrated in Pisum sativum meiosis. Pisum meiosis differs from the usual plant meiosis by: (1) the peculiar timing of the late prophase, (2) the existence of extrachromosomal fibrillar structures appearing at synizesis and staining by Feulgen - Light Green, and (3) the appearance of multiple nucleoli in the late prophase. The possibility that these nucleoli and fibrillar structures are of relevance in relation to gene amplification is discussed.