COMPARATIVE CYTOLOGY AND GENETICS OF ASYNAPTIC MUTANTS IN LYCOPERSICON ESCULENTUM MILL

Abstract

HE conjugation of the chromosomes during the first meiotic prophase T is referred to as synapsis. Therefore a lack of this prophase pairing is correctly referred to as asynapsis. However, in actual use the term asynapsis has been applied to a lack of chromosome pairing at any point during the first meiotic division. While this use of the term is not strictly in accord with the original definition it is probably best to use it in this sense unless one can positively determine that the lack of pairing at the later first division stages is not related to a lack of pairing at the time of chromosome conjugation. LI, PAO and LI (1945) have used the term desynapsis to describe the condition where chromosomes initially pair at pachytene but fail to remain paired at subsequent meiotic stages. This terminology is certainly more accurate if initial pairing is complete but is just as inaccurate as the term asynapsis unless it can be demonstrated that the lack of pairing at metaphase is greater than can be accounted for by the amount of non-pairing at pachytene. Without thorough examination of the early prophase stages it is impossible to tell whether it is more accurate to use asynapsis or desynapsis to designate lack of pairing occurring at later first division stages. The occurrence of some bivalents at the later first division stages does not necessarily indicate that all chromosomes were paired at the early prophase stages. Even the occurrence of complete metaphase pairing in some sporocytes does not necessarily indicate that complete pachytene pairing occurred in all sporocytes. On the other hand, the complete lack of metaphase pairing does not demand the complete lack of pachytene pairing. The prophase stages of many cases of asynapsis reported in the literature were not studied and therefore it is impossible, when considering these cases, to know which of the two terms, asynapsis or desynapsis, is more accurate. In the present work three of the mutants considered were not given extensive cytological study, whereas cyto-logical examination of the other two mutants indicates a variable amount of pachytene pairing. Therefore, throughout the paper the term asynapsis will refer to any lack of pairing occurring during the first meiotic division. Reported cases of asynapsis can be classified into the following groups. (1) Asynapsis in species hybrids. (2) Asynapsis caused by the loss of a chromosome pair. (3) Asynapsis in apomictic organisms. (4) Asynapsis induced by external conditions. (5) Asynapsis due to the action of a gene or