Critical evidence of parasynapsis in Oenothera

The formation, inOenothera species, of rings or chains of chromosomes joined end to end, first found by Gates (1908), was shown by Cleland (1922) to be characteristic for particular forms. InOe. Franciscana , for instance, there is at diakinesis and metaphase regularly a ring of 4 chromosomes and 5 bivalents, and inOe. Muricata (Cleland, 1926), there is a ring or chain of 14 chromosomes. Numerous workers have added to the number of species, mutants and hybrids examined, and Gerhard (1929) has presented a classified list. It is apparent, then, that everyOenothera form has a specific chromosomal configuration or, more precisley, that each form has particular configuration which is more frequent than any other and from which other possible configurations may be derived by simple modifications. Undoubtedly, therefore, there must be some innate reason for the fixity of construction, and Darlington (1929) traces it to interchanges of end-segments between non-homologous chromosomes occurring during the evolution of the various forms. Each species is thus a structural hybrid, true breeding by reason of the operation of a balanced lethal system. Pairing is essentially parasynaptic and takes place between homologous end-segments with subsequent terminalisation of chiasmata established between the paired segments. In this way, chromosome segments must be specific in their position and attraction, a matter that has been adequately proved by Blakeslee and Cleland (1930) and Cleland and Oehlkers (1930).