Chromosomal strategies for adaptation to univalency.

The orientation and segregation behaviour of different types of univalents, namely sex chromosomes, B chromosomes and autosomal univalents, were analysed in living spermatocytes of eight evolutionarily distant grasshopper species. The meiotic behaviour of each univalent was characterized in terms of velocity of prometaphase movements, frequency of reorientations, types of final orientation at metaphase I and modes of segregation at anaphase I. All these features were found to vary between different univalents. Certain combinations of these traits, defining a 'chromosomal strategy', appear commonly together in certain chromosome types, indicating that they are the result of selection acting on the chromosomes to increase transmission effectiveness. The sex univalents show in general a strategy in which all the features favouring an eventual equational segregation at anaphase I tend to be minimized. There is much more variation in behaviour among B chromosomes than among X chromosomes, which is a reflection of their heterogeneous nature. Induced autosomal univalents are studied in Locusta migratoria. They show a very irregular behaviour, indicating their lack of adaptation to univalency.