Notes on the Cytology and Genetics of the Genus Fuchsia

NOTES ON THE CYTOLOGY AND GENETICS OF THE GENUS FUCHSIA. BY KUDOLPH BEEK, B.Sc,., F.L.S. (Witch Plates XXII--XXIV.) IN contrast to the extensive literature which has grown up round the cytology and genetics of Oertothera very little work has been done upon these subjects in the case of the Fuchsias. In 1850 Wimmel(17) called attention to the irregularities in the number and size of the pollen grains produced fi'om the mother-cell of Fuchsia and in 1886 Wille (16) made further observations upon the same subject. He counted five to fourteen mierospores arising from a single mother-cell. He ex- plained the occurrence of these supernumerary pollen grains by assuming that " In dem Falle, we bei Fuchsia sp. sieben Zellen entstanden waren, konnte ieh nieht dariiber im Zweifel sein, das diese daher kam, dass drei der Zellkeme der Tetrade noeh sieh einmaI getheilt hatten, ehe die Cellulosequerws ausgebildet waren, ws der Vierte ungetheilt blieb"(16 p. 61). Where five or six mierospores were produced he believed the explanation to be similar. In the case of the higher numbers (e.g. 14,) he was unable to follow the cell-divisions, and was uncertain whether the additional pollen grains are due to secondary divisions of one pollen mother-cell alone or whether they are to be derived fi'om two or more primitive mother-cells which have not become separated from one "another in the usual manner. In 1907 the present writer(I) published a short note upon "The Supernumerary Pollen Grains of Fuchsia and their mode of development." It was found that the abnormal numbers of pollen grains which develop fi'om the mother-cells is due to the irregular distribution of the chromosomes during the first meiotic division and that no evidence could be found to support Wille's explanation of the phenomenon. In 1912 Bonnet (8) published an account of the tapetal cells of certain Angiosperms and included those of Fuchsia among the number. Beyond this I can find no literature dealing with the cytology of Fuchsias. 214 Cytology and Genetics of the Genus Fuchsia Some time ago I continued my studies of the cytological features which are connected with the development of supernumerary pollen grains in certain species and hybrids of Fuchsia and concurrently I began a series of observations upon the genetics of the genus. Circumstances have intervened and prevented the completion of the work, but it was thought that it might be useihl to future workers if a brief statement were made of such results as were obtained, and of the lines of work which appeared to promise interesting results. A. Cytology of Fud~sias. I will first describe the facts which are to be seen in a hybrid form known as "Alice Hoffman." The early stages of the first meiotic division in the pollen sacs take place in a normal manner as will be seen fi'om P1. XXII, figs. 1--8. The spindle which develops is apolar and the chromosomes become arranged regularly at its equator at the conclusion of the prophase. During the anaphase, however, the chromosomes move very irre- gularly towards the spindle poles and some, either singly or in groups, lag behind the rest, and often become entirely cut off from the two main chromosome groups. This irregular distribution of the chromosomes is shown in figs. 9 and 10. At the conclusion of the division these scattered and separated chromosomes behave variously. In the majority of cases distinct nuclei are developed in association with them. In some instances only a single chromosome may become organised into a smatl nucleus, but more commonly small groups of two or more chromosomes are associated together in the development of a nucleus. The size of the nucleus which is formed depends upon the number of chromosomes which enter into it. Examples of such nuclei can be seen in figs. 11 ~md 12. In several cases the chromosomes were so scattered at the beginning of the telophase that they did not lead to the formation of a number of separate nuclei, but they all became ineludod within a single, large, irregular nucleus. In fig. 12 one small nucleus is seen to have developed round a chromosome which had become widely separated fi'om its fellows, whilst all the remaining chromosomes were included in the large irregular nuclear body which occupies the centre of the mother-cell. Whilst it is more usual to find nuclei organised rmmd the scattered chromosomes there are other instances in which these bodies degenerate without giving rise to a nucleus. In fig. 13 the chromosomes excluded from the two principal groups are probably about to undergo degeneration. All the nuclei, both small and large alike, enter upon the RV])0LPH BEE~ 215 second meiotic division in the usual mamier. Spindles, varying in size with the number of chromosomes, are developed in association with each nucleus, and the course of mitosis proceeds in quite a normal manner No irregularities in the distribution of the danghter-chronlosomes are to be seen in this division. PI. XXII, fig. 17 gives a representation of the second meiotic division. Single chromosomes each associated with a small spindle can be seen in fig. 14. In Pl. XXIII, fig. 16 we have a case whieli probably represents the second division of one of the large, irregular nuclei referred to above and represented in fig. 12. The nuclear divisions wh:ich have been described in the foregoing account are followed by the division of the cells so that each nucleus becomes associated with a distinct cell. Pl. XXIII, fig. 19 represents a mother-cell which has divided up in this manner to form the fiStials of a number of pollen grains. In fig. 18 ten pollen grains will be seen to have developed fi'om a single mother-cell. It is readily apparent that the size of the pollen cell is dependent upon the size of the nucleus with which it is associated and this, in turn, depends upon the number of chromosomes which enter into its composition. The details of this relationship will not, however, be dealt with in the present paper. From what hasbeen said above it will be seen that no evidence was found of the existence of secondary divisions of the cells such as Wille described, nor do the presen~ observations give any support to this author's suggestion that a fusion (or non-separation) of primitive mother- cells might occur in those cases in which the supernumerary microspores were very numerous. In addition to the variety "Alice Hoffman" the meiotic divisions of Fachsia globosa and Fuchsia corallina were studied in detail. In both these plants more than the normal number of four microspores are produced fl'om each pollen mother-cell. It was found that the distribution of the chromosomes during the first meiotic division is quite similar to that taking place in" Alice Hoffman" and that single chromosomes, or small groups of them, become separated from the rest, and usually give rise to small nuclei. Here also some of the isolated chromosomes may fail to organise nuclei, but undergo degeneration instead. It will be un- necessary to describe the details of meiosis in these two forms, but it will be sufficient to call attention to P1. XXIII, figs. 20--2~ which represent the principal facts of interest. 216 Cytology and Genetics @the Genus Fuchsia The pollen grains of Fuchsias tbllow the general Onagraceous type. Their membrane consists of au exospore, a mesospore, and an endospore, and is furnished with two o1" more interstitial bodies. The Fuchsias have been divided into two groups according to whether" their pollen grains possess two or three interstitial bodies. Fuchsia globosa is a typical example of the group which possesses three interstitial bodies upon each pollen grain, whilst F~ohsiaprocumbens has pollen grains with only two interstitial bodies. Whilst these numbers are characteristic of the majority of fet'tile pollen grains in each case, yet they are not constant in those instances in which ilTegular pollen development takes place. In these cases the number of interstitial bodies appears to vary with the size of the pollen graiu, and in some of the smaller gl~ins only a single interstitial body is forlned. Fig. 27 shows such a small, supernumerary pollen grain with only a single interstitial body. Although not shown in this figure, these snlal] pollen grains, apart fi'oln the number of interstitial bodies they possess, have membranes which are identical in Stl"UCtUl"e and chemical composition with those of the larger grains. We have already seen that the distribution of the chromosolnes to the pollen gl"~ins is an irregular one, and that the small grains receive only a small proportion of their normal number of chl"omosornes. Notwithstanding this all the polle11 grains develop walls which are characteristic of the genus both in structure and chemical composition. These facts have an interesting bearing upon the theory of the localisa- tion of generic and specific characters ill particular chromosomes, since the chromosomes which any particular pollen grain receives is perfectly haphazard in the present instance. The explanation is probably similar to the one which has been suggested in the case of the development of certain anilnal eggs in which the cytoplasm becomes set to a definite line of development at an early stage. We may probably assume that the cytoplasm of the pollen mother-cell has already been set, through the influence of the still undivided nucleus, to a definite course of development, and that it already has the mechanism implanted in it for the formation of pollen membranes of a definite structural and chemical constitution.

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