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ON THE CYTOLOGY OF TITYUS BAHIENSIS WITH SPECIAL REFERENCE TO MEIOTIC PROPHASE F. G. BRIEGER AND E. A. GRANER Department of Genetics, Escola Superior de Agricultura “Luiz de Queiroz,” Universidade de S& Paulo, Brasil Received November 27, 1942 ITYUS BAHIENSIS, the common scorpion in the region of Piracicaba, TSgo Paulo, Brazil, differs considerably with regard to form and number of chromosomes from related species described by other authors, as first stated by PIZA(1939a). Somatic divisions show six irregularly curved chromo- somes: in the meiosis of spermatocytes three tetrads are easily distinguished, all chromatids being rod shaped, without showing signs of chiasmas, of con- strictions, or any specific attachment region. The most interesting feature so far reported is the parallel movement of the chromosomes or chromatid pairs during the first anaphase. This parallel movement in Tityus seems to be the same as described first by SCHRADER (1935) for some Hemiptera (see GEITLER1938 for further references). PIZA (1939b) has formulated far reaching speculations on this subject with which we cannot agree. The description of meiotic prophase as given by PIZA(1939a) is very incom- plete. He describes very briefly leptotene, zygotene, and early pachytene stages, without corresponding illustrations. In the description of pachynema PIZA(1939”~ p. 256) writes that “the nucleus seems to be occupied by extensive filaments of very clear chromomeric structure which describe many and large spirals, strongly accentuated in the point of separation of the respective seg- ments.” It is not clear to what structure the term “segment” refers. Later prophase stages were not observed in the three males studied by PIZA.He assumes that in late pachynema the tetrads condense and become regular in outline, passing thus into metaphase (fig. 3, Z.C.). Finally PIZAdeclares cate- gorically that “nothing exists which could be compared to a true chiasma” (Z.C., p. 257). Here again we encounter some difficulty in interpreting the state- ment without a clear definition of what the author calls a “true chiasma” (quiasma verdadeiro). Figures d and f (plate I, Z.c,), which show photomicro- graphs of a chromosome arrangement in metaphase with a reciprocal translo- cation, are described as “chiasmas between two bivalents caused by reciprocal translocations.” We could not find any further observations on the prophase in the later papers (PIZA1939b, 1940, 1941). The first difficulty in our study of this animal consisted in the relatively small number of males to be encountered. In several localities no male was found at all among several hundreds of females, and it has not yet been possible to decide about the method of reproduction in these cases. In other localities, all in the vicinity of the same town, males were found, though always in smaller number than females. Small individuals have generally very few cells in meiotic GENETICSa8: 269 July 1943 270 F. G. BRIEGER AND E. A. GRANER division and these are often all in the same phase, while larger males, which are rarer still, show a higher rate of division. While leptonema and metaphase are frequently encountered, the intermedi- ate stages are rare, and this paper is based essentially on the observations of about half a dozen males within some 30 studied. But whenever some phases were found in an animal, they were then very frequent. All possible care was taken, by observing the general characters of the cell, of the spindle, of the chromosome structure and size, to be certain that the different stages observed were arranged in the proper sequence and that no essential phases were over- looked. The photomicrographs which accompany this paper were taken from aceto- carmine smears which gave the clearest pictures. In order to have all important structures more or less in one plane for photography, intact cells were flat- tened by slight pressure on the cover glass. Leptonema, while frequently encountered, is impossible to analyze in detail, owing to the extreme length of the six fine threads which are densely inter- woven. Zygotene pairing proceeds normally. Figure I shows two cells in which pairing seems to be completed. In at least one of these, as shown by the arrow, the three pachytene pairs are visible. As is the rule in meiosis, three processes follow early pachynema: contraction by coiling or otherwise of the chromonemata, separation and consequent visibility of the four constituent chromatids, and the opening out of loops during diplo- nema. All these processes are clearly visible in Tityus (fig. 2-4), but diplonema leads to a complete separation of chromatid pairs. This may be seen, for in- stance, in the cell of figure 2 indicated by an arrow. One tetrad cannot be analyzed, the second tetrad has a typical diplotene structure with five loops in alternating planes, while the third tetrad is completely separated into two chromatid pairs. Another case of opening out of loops is seen in figure 4. Here one tetrad is already completely separated into chromatid pairs, another opens up in the middle region, both ends still being closely paired, and the third tetrad seems united only in one point at a distance about one-fourth of the total length from its end. The chromatid structure is here especially clearly visible. Finally at the end of diplonema, we encounter six double threads in each cell (fig. 5) showing to some degree a tendency to associate in three pairs, but without any sign of the preceding actual and intimate pairing. At this stage, the shortening of the chromatid pairs is already considerable, but has not quite reached its maximum. We cannot say whether or not crossing over occurs at the points where diplo- tene loops change their planes and which conventionally are called chiasmas (see fig. 3). A genetical verification of the occurrence of crossing over or its ab- sence is at present impossible in Tityus since no mendelian characters are known and since breeding experiments with this highly dangerous and poison- ous animal do not seem advisable. Thus it is not possible in Tityus, where the number of seeming chiasmata is reduced between diplonema and metaphase, 3 7 PL:An: I 1'hotomicroRral)hs of aceto-carmine smears; magnification 3000X. FIGI.RE1.-I'achynema. I'IGVRES 2 to 4.--Successive stages in diplonema. FK:I.RE5.- Late diplonema, showiny: separation of homolo~ues. FIGL.R~:6.- I.:arly metaphase. I.'IG[.RE 7.- Late metaphase. I'ICI,RE 8.- Sl"nato~onia1 metaphase. CYTOLOGY OF TITYUS BAHIENSIS 271 to say whether we are concerned with true chiasmata or to the unravelling of relationally coiled chromosomes. No paraphase (pro-metaphase) stages have been found as yet, and the next stage encountered is metaphase. Taking into consideration the size and form of the chromatid pairs in the latest prophase and the earliest metaphase stages, it seems to us doubtful that any important stage is missing between those il- lustrated by figures 5 and 6. We may suppose that there is no pronounced dia- kinesis, since at the end of diplonema the chromosomes have already reached nearly the degree of contraction characteristic of metaphase. Of course there must exist a paraphase (pro-metaphase) during which the chromosomes or chromatid pairs move not only to the equatorial region, but begin or even com- plete their second pairing. We find invariably at metaphase three tetrads with chromosomes parallely paired, which may or may not show an intimate association at both ends, imitating thus terminalized chiasmas. No loops may be seen, only occasionally an unravelling of a chromatid spiral as in the lower right corner of figure 6. In some instances the more or less unravelled chro- matid, only loosely and irregularly coiled, may be seen very clearly. From the general appearance and its not too frequent occurrence we feel convinced that this unravelling is of no especial significance and may be a “vital artefact” or may be caused by the fixative. More or less complete unravelling of chromatid coils is not rare in organisms with large chromosomes such as Hyacinthus, Rhoeo, and Tradescantia. Similar structures may be seen in the half schemati- cal drawings by PIZA(1939a, fig. gb, c, d), who offers no definite explanation except to reject the idea that they may be caused by an action of the spindle fibers. Later metaphases in polar views (fig. 7) show the chromatids well sepa- rated, strictly parallel, except for small terminal portions in some cases. Side views, which are difficult to photograph, show clearly and equidistant the ends of all four chromatids of each tetrad. This aspect is very similar to the sche- matical drawing of HUGHES-SCHRADERand RIS (1941, fig. 3), except that in Tityus there are not two, but three groups of four chromatids, each corre- sponding to a tetrad. Thus, there can hardly be any doubt that two types of pairing occur: (I) normal zygotene pairing which subsequently disappears during diplonema and (2) metaphase pairing. Metaphase pairing has evidently no relation to the zygotene pairing. As a tentative and provisional hypothesis we may assume that it is caused by forces similar to those which cause somatic pairing, though the latter is never so intimate as in the present metaphases. It is difficult to understand how the four chromatids of each tetrad remain so closely parallel and equidistant during a stage which seems to persist during a relatively long time, judging by the frequency with which these metaphases are found. No connecting substance can be seen in the preparations, and it is useless to speculate about a responsible “force.” In order to test whether somatic pairing normally occurs, we studied sperma- togonial divisions. There is some indication that the six chromosomes, all of which are of equal size and form, have a certain tendency to associate loosely 272 F.

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