Chromosome Numbers in Cimex Such a Suggestion Is As Useless As Mather's

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Chromosome Numbers in Cimex Such a Suggestion Is As Useless As Mather's 358 NATURE AUGUST 20, 1938, VoL. 142 differ profoundly according to the intensity of of which can be performed by the remainder of the Feulgen's reaction. The proximal ends of the auto­ complement, or are genetically inert. somes continue to unite with the heterochromatin All C. lectularius material so far examined has region of the X -chromosome in the same way as yielded a haploid autosome count of twenty. Since they united formerly with the chromocentre. these three karyotypes form a series having chromo­ When the chromosome bands begin to form, the some numbers which are multiples of four, there is heterochromatin region extends still more and the to suppose that (a) the genus is polyploid in small number of chromomeres of early stages dis­ ortgm, (b) the stem number in Cimex is four and not integrate lengthwise into a greater number. When six as was suggested by Slack1 and Vandel2 for the the genonemata become double, the number of Heteroptera. H. D. SLACK. chromomeres in the row is also doubled in both University, euchromatin and heterochromatin chromosome Glasgow. regions. Thm; a typical picture of the inert region of June 28. the X-chromosome of D. funebris in the salivary 1 Slack, H. D., British Association, Nottingham (1937) (nnpnblished). gland nuclei of large larvm is obtained. 'Vande!, A., Proc. Zoo!. Soc., A, 107, 519 (1937). When the bands in the salivary gland chromosomes of D. mel<Snogaster are formed, the round chromocentre of a resting nucleus divides into parts belonging to Time Sequence of Crossing-Over separate chromosomes. Th6ile chromosome parts 1 2 divide into chromomeres later on. MATHER has answered the challenge of Charles regarding the time sequence of crossing-over. Like In D. repleta the double-armed X-chromosome is 3 represented only by one banded chromosome in the Charles, I feel sceptical of Mather's evidence that salivary gland nuclei. Thi'! can be explained only by crossing-over begins near the centromere. the fact that the second inert arm forms the chromo­ Mather suggests that the experiments on the centre, which in the large salivary gland nuclei does effect of age, temperature and inversions in Drosophila not differ externally from the chromocentre of the show the differential effect between the centromere resting nuclei. In other words, this arm retains the and the ends of the chromosome to be expected on same compact condition as in the resting nucleus. his hypothesis. Before he can use this evidence as a In the large salivary gland nuclei of D. virilis and support for his hypothesis, he must first show that the D. robusta, the chromocentre of a resting nucleus is genetically unsplit region near the centromere be­ retained as in D. repleta, but is decreased in dimen­ haves similarly to the ends of the chromosomes in sions. This can be explained by the fact that in all other respects than crossing-over. It is perfectly young larvm small parts separate from the chromo­ reasonable to suppose that the genetically unsplit centre and disintegrate into chromomeres. Later on, region and its neighbourhood react quite differently the number of chromomer6il doubles exactly in the to external influences. Indeed there is much evidence same way as in the euchromatin chromosome regions. for this, but it is not necessary to assume, as Mather The remaining part of the chromocentre represents a does, that the observed changes in linkage are due fusion of small parts of heterochromatin, proximal to the fact that the first formed chiasma is proximal ends of all the chromosomes in D. robusta and in D. to the centromere. If, for example, an increase in virilis all the heterochromatin of chromo<;ome III in temperature influences the distance at which the addition. chromosome is genetically split at the time of crossing­ The differences of the structure of the point of over, the changes in linkage values will be observed union of the proximal ends of all the chromosomes in no matter where crossing-over started on the the salivary gland nuclei of large larvm of different chromosome. '!pecie'! of Drosophila can thus be explained, whether The precocious splitting in unpaired parts of trivalent the whole or a section of the chromocentre of a or univalent chromosomes provides more factual evi­ resting nuclei is transformed into the inert chromo­ dence than that adduced by Mather. The papers by some regions, that is, obtains a chromomere structure. Charles, Schweitzer and Mather are most useful in The thread leading from the chromocentre to the suggesting novel modes of attack; but indicate the nucleus is formed during the transition from telophase dangers of jumping to conclusions, however reason­ to the resting nucleus. Its nature has so far not been able, which are derived from the cytological or statistical methods at present favoured by many. made clear. FROJ,OVA. s. Until the mechanics of chromosome pairing are Institute of Experimental Biology, understood, it is difficult to utilize the data of pairing Moscow. June 17. in structurally changed forms in the way Mather has 1 Frolova, NATURE, 141, 1015 (1938). done. A juxtaposition of the centromeres at early 'Neitz, Biol. Zol., 54, 11/12 (1934). meiotic prophase as seen in salivary glands would 'Heitz, Z. ZeUJ. u. micr. Anat., 19 (1933). account for the behaviour of the heterozygote of the Delta 49 inversion, but without factual evidence Chromosome Numbers in Cimex such a suggestion is as useless as Mather's. It is A SUB-SPECIES of Cimex, phenotypically inter­ possible that Mather is correct in his assumption mediate between C. lectularius and C. columbarius regarding this theory, but more genetical evidence is and obtained from laboratory white rats, has been required before acceptance is possible. found to comprise two forms differing in the number F. w. SANSOME. of chromosomes. In most cases the haploid number Botanical Department, of autosomes is twenty-four, but in occasional University of Manchester, specimens it is sixteen. No variation in phenotype, Manchester, 13. correlated with thi.,; change in number, has been July 25. observed, nor is the normal course of meiosis affected. 1 Mather, K., NATURE, 142, 157-158 (1938). This suggests that eight autosomes in this sub­ • Charles, D. R., J. Genet, 36, 103-26 (1938). either represent a duplicate set the functions 3 Mather, K., J. Genet., 33, 207-35 (1936). © 1938 Nature Publishing Group.
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