Chromosome Studies in Some Dioecious Plants, with Special Reference to the Allosomes

Abstract

1. The chromosomes of the male plants of 17 genera, 22 species and 2 varieties of dioecious phanerogams have been investigated. Of these the following 13 forms show each an unequal pair of chromosomes in addition to autosome pairs at the meiotic division in microsporocytes. This unequal pair of chromosomes is assumed to be a sex chromosome complex of an XY-type. Consequently in these forms the male plants are heterogametic with respect to sex. The 13 forms are Salix leucopithecia, S. sachalinensis, S. japonica, S. melanostachys, S. gracilistyla, S. viminalis var. yezonensis Morus bombycis, Cannabis sativa, Datisca cannabina, Daphniphyllum macropodum, Trichosanthes japonica, Hydrilla verlicillata and Trachycarpus excelsus.2. In Cudrania triloba, Acer negundo, Trachycarpus excelsus, var. Fortunei and Ginkgo biloba, one unequal pair-like pair is found at the first meiotic metaphase in microsporocytes; but it is not safe to take this for a true unequal pair of chromosomes until a further examination has been made.3. One large chromosome pair in Morus and Trachycarpus often divides in two unequal parts at the first meiotic division. The signifi-cance of this particular behaviour, especially in relation to sex determi-nation, is not yet known.4. Although many suitable figures of metaphase and anaphase of the first and second meiotic divisions in the male of Spinacia oleracea have been examined, no evidence of the existence of sex chromosomes has been obtained. An examination of the female has still to be undertaken.In microsporocytes of Aucuba japonica, 16 chromosomes are found at the first meiotic anaphase. The zygotic numbers of chromosomes in both sexes are the same, being 32. No evidence as to the sex chromo-somes could be obtained in the male plant.5. All Salix plants studied, except one form of S. sachalinensis, have 19 as the gametic chromosome number which is a basis in Salicaceae. The meiotic division is quite normal in them.One form of S. sachalinensis from Hokkaidoô has ca. 24 chromosomes at the first meiotic metaphase and their behaviour in the meiotic stages is irregular.6. Humulus japonicus, growing wild in the vicinity of Tokyo, has a tripartite chromosome in addition to 7 autosomic gemini at the first meiotic division in microsporocytes. At the first meiotic metaphase, the tripartite chromosome divides in such a way that the two end chromosomes go to the one pole, while the middle one goes to the other. Its behaviour is strikingly similar to that of Rumex acetosa. As a result, with respect to chromosomes, two kinds of pollen grains maybe formed. These results confirm that attained by KIHARA.7. Humulus lupulus has 10 gametic and 20 zygotic chromosomes. In the first meiotic division of microsporocytes, 16 chromosomes of the 20 form 8 gemini in all, while the 4 remaining chromosomes do not form gemini, but are connected end to end to form a beaded string. This tetrapartite chromosome can be identified in several stages from early diaphase, in the first meiotic division. At metaphase each alternate chromosome of the tetrapartite goes to opposite poles respectively. Thus the daughter nuclei receive an equal number of chromosomes, i.e. 10 respectively. The two middle members of a tetrapartite chromosome are equal in size and larger than the two end ones which differ in size from each other. As a result, two kinds of gametes may be formed, one having a larger amount of chromatin volume than the other. The tetrapartite chromosome may be a sex chromosome complex in H. lupulus, a new type of sex chromosome.8. At metaphase of the first meiotic division in the microsporocytes of Xanthoxylum piperitum, 35 chromosomes are counted, one of which is not only the largest