Studies on synapsis. II.—Parallel conjugation and the prophase complex in Periplaneta with special reference to the premeiotic telophase

Abstract

The reality of synapsis, or the pairing of chromosomes of biparental origin preparatory to their segregation during the maturation divisions of the germ cells, provides at present the most fruitful basis for any attempt to correlate genetic phenomena with cell structure; and it rests upon data derived from sources that are strictly independent of the behaviour of the chromosomes during the remarkable series of events intercalated in the prophase of the heterotype mitosis. It is, however, only possible to gain a knowledge regarding the manner in which homologous chromosomes are brought into association with one another by a study of the meiotic phase itself. To the earlier workers the reality of synapsis was the all-absorbing problem, while the means by which it is effected formed a secondary consideration. It is possible, given the conjugation of chromosomes derived from alternate parents, to explain the independent segregation of allelomorphic pairs on the assumption that their material basis resides in different bivalents; while their localisation in identical bivalents affords an equally satisfactory interpretation of the phenomena of coupling and repulsion. Indeed were coupling and repulsion always absolute the mode of synapsis would not present an urgent cause for enquiry. The progress made of late years, however, in the investigation of partial linkages, makes it imperative for the further development of the chromosome hypothesis to postulate some mechanism for the exchange of chromatin particles between conjugating elements; so that the degree of intimacy involved in the synaptic union assumes a new significance. While recognising with the utmost respect the brilliant speculations of Prof. Morgan and his school, it cannot be truthfully stated that there exists among cytologists at present a very high degree of unanimity as to the line along which the solution of this question must lie. In recent years the belief in telosynapsis (end-to-end union) has increased in prestige among botanical cytologists, while the alternative interpretation of parasynapsis (parallel conjugation) has found greater favour among those who have studied animal forms. In attempting to interpret satisfactorily the events of the meiotic phase in the case of animals, interest centres pre-eminently in those phenomena which intervene between the last premeiotic telophase and the formation of the “bouquet” on the one hand, and the process by which the reduced and longitudinally split (diplotene) segments become transformed into heterotype chromosomes on the other. The former furnish direct evidence of the relation of the meiotic filaments to the somatic chromosomes, while the latter in the light of the subsequent mitotic processes afford indirect data regarding the mode of synapsis, the reality of synapsis being assumed. Regarding the first, to an impartial student the work of the Schreiners (1) 1906, Jannsens (2) 1909, Agar (3) 1911, and Wilson (4) 1912, based in each case on particularly suitable material as regards both size and seriation, cannot fail to carry the conviction that here at least there is a fusion of whole chromosomes parasyndetically; and such an interpretation is fully consonant with the usual derivation of the heterotype chromosomes by separation along the line of cleavage in the diplotene filaments. There are, however, particular cases, notably among Flatworms and Orthoptera, where certain authors (Goldschmidt (5), Sutton(31), Davis (32), and Buchner (6),e. g.) have described the genesis of the tetrads in a manner which can only be reconciled with the theory of synapsis on the assumption that the conjugating elements are united by their extremities alone. In the case of plants the position is somewhat different; here, according to the most recent and very lucid account of Miss Digby for Osmunda, provision is made for the conjugation of univalents in the second contraction, while the fusion witnessed in the first contraction figures involves the reassociation of half chromosomes split in preparation for an arrested division in the preceding telophase. The chief issue between the rival schools in this case concerns not primarily that suggested by the etymological distinction, but the significance of the earlier stages in the meiotic phase. The telosynaptists interpret the first contraction in terms of the premeiotic prophase, whereas the parasynaptic school, like Grégoire (7), attempt to compare it with the bouquet zygotene processes in animals.