Megaspore development in Oenothera rubricalyx, with a note on chromosome linkage in Oenothera angustissima

It has long keen known (Gates, 1908) that at meiosis, in the pollen mother-cells of Oenothera, the chromosomes do not of necessity become paired, but may remain joined together in chains. Of recent years it has transpired that the amount of pairing and chromosome linkage is constant within each species and each mutant. Morphologically different hybrids resulting from a single cross may differ in the amount of linkage exhibited, but within each hybrid type the linkage is constant. Reciprocal hybrids may differ from one another morphologically and cytologically, but each form may breed true if a sufficient number of chromosomes are linked; for, once determined, the amount of linkage in any form is unvarying. In certain other genera of plants various linkages of chromosomes have been found, but Oenothera is at present unique in the degree and constancy of linkage observed. The view has therefore been expressed (Gates, 1928) that the small-dowered species, nearly all of which show complete chromosome linkage, breed true on account of this linkage and are really permanent hybrids, the formation of a ring of 14 chromosomes resulting from the bringing together of non-homologous chromosomes in the fertilization nucleus. Before any very definite conclusions could be drawn from these results it was necessary to find out whether the linkage, which is evident in the nucleus of the pollen mother-cells, also prevails in the nucleus of the embryo-sac mother-cell at reduction division. From an early paper of Davis onOe. biennis (1911) it was obvious that some suck linkage does occur on the female side. The present work was undertaken primarily to determine whether similar linkages occur in the two types of mother-cells of a single plant. Before the work was completed Håkansson (1928) showed that the process of reduction is similar on the male and female side inOe. Lamarckiana and several of its derivatives.