1942 Volume 13 Issue 1 Pages 30-41
(1) In the present paper, the author summarised the result of a chromosome number counting, which was made by him on the principal members of Citrus species, Fortunella species and Citrus hybrids. In most of the cases the counting was made, by smear method, on the chromosomes at meiotic division of the pollen motherr cells.
(2) Some one hundred and fifty kinds of Citrus, which represent subgenera, sections and subsections of the genus, excluding Sect. Papeda, has proved that they are all diploid, having 9 chromosomes at the haploid, 18 at the diploid phases (Table 1; Figs. 1-63; Pl. I, 1-7, 11)
(3) Throughout the course of the experiment, the author obtained only two examples of tetraploidy among the citrus fruits: One is the Shikinari mikan, a variety of C. madurensis, and the other is the Sampson tangero TUH 186 (Figs. 64-67, Pl. I, 8). As to the charactor of the plant, the latter is somewhat different from the typical Sampson tangero which is a hybrid between C. Tangerina and C. paradisi.
(4) The Temple orange, a suporsed species hybrid between C. sinensis and C. Tanqerina, is diploid and this plant conducts regular meiosis. Similar behaviour is also observed in the Tanikawa buntan, a species hybrid which was originated at the Imperial Horticultural Experiment Station at Okitsu by crossing C. grandis and a certain Citrus species belonging to Sect. Aurantium. Both plants form 9 gemini at the first metaphase, indicating a high degree of affinity (Figs. 68-69).
(5) Diploid constitution and high affinity of the chromosomes were shown by generic hybrid, such as the Rusk citrange (C. sinensis×Poncirus trifoliata), the Eustis limequat (Fortunella japonica×C. aurantifolia), citradia (Poncirus trifoliata×C. Aurantium) and the Thomasville citrangequat (Fortunella margarita×C. sinensis×Poncirus trifoliata)(Figs. 70-73). With the exception of the Eustis limequat, the meiosis observed in thesehybrids was practically normal.
(6) Poncirus trifoliata and four species of Fartunella, including F. Hindsii, were determined to have 9 chromosomes in the nuclei of the garnetic cells (Fig. 57, Table 2).
(7) With reference to the results of the present study as well as the works of the investigators who counted chromosome numbers of citrus fruits, the author came to the conclusion that, in the genus Citrus, diploidy is very prominent, while poliploidy is very rare. This fact suggests that the evolution of the species, the standard varieties and forms in this genus is mainly brought about by chance seedling and bud variation, due to gene mutation, or by recombination of allelomorphs which may be induced by hybridization; and that, in this respect, polyploidy has played, hitherto, no important part.
(8) The prevalence of affinity of chromosomes demonstrated by the species hybrids, as well as, by the generic hybrids, has proved that genera Poncirus, Citrus and Fortunella are represented by the same genom. This cytological conclusion presents an evidence to the opinion of systematists who believes that these genera are linked closely in the phylogenetic relationship.