Homologous Autosomes Research Articles

Two Heterozygous Females of Mus (Leggada) minutoides musculoides Temm. with a Partially Deleted X Chromosome Arm

In his previous publications (Matthey, 1963–1966), the author has studied 26 individuals of the pigmy-mouse, Mus (Leggada) minutoides musculoides Temm., from different countries of Africa. This subspecies appears as a polymorphic Robertsonian system: the diploid numbers are equal to 22, 31, 32, 33, 34, while the number of major arms (N.F. = fundamental number) is constant, i.e., 36. The X chromosome is large and nearly metacentric, the Y large and submetacentric. It is demonstrated that these sex chromosomes are formed of an acrocentric X and Y (like those of the domestic mouse) translocated onto two homologous autosomes, which, at meiosis, furnish the pairing segments associated through a true chiasma. No variation of this pattern has been detected. In another species, Mus (Leggada) triton Th., the X chromosome is submetacentric (centromeric index, approximately 0.33), the Y short and acrocentric. In a sample from Bukavu (Congo), 11 females out of 21 show only one typical X, the other X is acrocentric and its length is equal to the length of the long arm of a normal X. The deletion of the short arm is also total or subtotal. No deletion occurs in the 18 males. Recently, five musculoides were obtained from the Ivory Coast. The three males, with diploid numbers of 34, 32, 32, are very similar to seven other specimens of the same country. Both females (2<i>N</i> =32) show only one normal metacentric X chromosome. The other X is submetacentric (I.C. = approximately 0.26) and appears to have lost the major part of one arm. Because of insufficient material, it is not possible to establish whether females homozygous for this deletion and males with the sex chromosome constitution X<sub>d</sub>-Y occur in the population.

An Autoradiographic Study of DNA Synthesis in the Sex Chromosomes of Two Marsupials with an XX/XY<sub>1</sub>Y<sub>2</sub>Sex Chromosome Mechanism

<i>Potorous tridactylus</i> and <i>Protemnodon bicolor</i> are marsupial species in which the sex chromosome system is XX/XY<sub>1</sub>Y<sub>2</sub>. Nearly all the long arm of the X is homologous with the Y<sub>2</sub>; the short arm of the X is unpaired in the male. During the last part of DNA synthesis in females only the short arm of one of the 2 X-chromosomes shows asynchronous synthesis of DNA. The long arm sof the X’s and homologous autosomes synthesize DNA synchronously. Some pairs of non-homologous autosomes synthesize asynchronously. The implications of this in terms of the Lyon hypothesis and the evolution of X-chromosomes are discussed.

FURTHER STUDIES ON THE PATTERN OF CHROMOSOME DUPLICATION IN CULTURED MAMMALIAN LEUCOCYTES.

Autoradiographic studies on chromosome duplication in cultured mammalian cells (have revealed several very significant findings, including the detection of asyncronous DNA replication in homologous X-chromosomes. In cultured cells froin ,a female Chinese hamster, Taylor (1960) observed that the short arm of one of the X-chromosomes terminates duplication during the first half of the D N A synthetic period, but rhe long arm of the same X-chromosome and the entire portion of the other X-chromosome duplicates only in rhe later part of the D N A synthetic period. A somewhat similar type of out-ofphase D N A replication has also been observed very clearly in presumptive X-chromoson~es in cultured human cells (Ailorishima et nl., 1962; German, 1962; Rowley et nl., 1963). In cultured cells from normal females as well as in diploid polysomic-X cells only one X-chromosome conlpletes its replication along with the autosomes, while the remaining X-chromosome or chromosomes continue their replication after rhe rest of the conlpleillent has terminated duplication. Although the morphologic identification of the X-chron~osomes in Chinese hamster cells is conlparatively easy, it is not so for human X-chromosoines. Since no late-replicating chromosome in group 6-12 + X is seen in normal males as it is in normal females, such late-replicating chron~osomes are considered to be X-chromosomes. T,he sex cl~romarin lnasses found in interphase nuclei with lnore than one X-~hromosome are thought to be forined b) the heteropycnosis of the Xchromosomes (Ohno et nl., 1959), and it is further believed that the heteropycnotic-X-chronlosonles are the ones which complete their D N A replication later than all other chromoson~es in the colnplement (Grumbach et nl., 1963; Atkins et nl., 1962). In general, the number of late-labelling X-chroinosornes observed has been equal to the maximum number of sex chromatin masses present in inter~hase nuclei and one less than the number of X-chroi~losomes in the L diploid complement. Based on the evidence from mouse genetics, Lyon (1961) has postulated that the heteropycnotic X-chroil~osome found in normal mammalian females is genetically inactive, and this ll~ypothesis is supported by cytological observations in cells with an X-autosome translocation in mice (Ohno and Cattanach, 1962) as well as by less direct evidence of glucose-6 phosplhate dehydrogenase activity in inan (Beutler et al., 1962). Pooling all the available information, it appears now that the X-chromosome which forms the sex chromatin mass in interphase nuclei, the heteropycnotic-X, the latereplicating-X and the inactive X are indeed the different manifestations of the same X-chromosome. In mammalian chromosome comuleinents. the X-chromosomes are not rhe I , only ones which show a definite Dattern of D N A re~lication. The autosomes I I and the Y-chromosome also appear to follow a ,definite time and morphologic sequence, and even homologous autosomes may show characteristically differ-

Studies on the Murine Chromosomes, IV

The number of chromosomes in the mole-rat, Nesokia nemorivaga taiwanus TOKUDA was found 46 in diploid and 23 in haploid. The diploid complement consists of 11 homologous pairs of V-shaped (syntelomitic) elements, 11 homologous rod-shaped (orthotelomitic) ones, and an unequal pair made up of a large X and a small Y element. The harvest-mouse, Micromys minutes takasagoensis TOKUDA possesses 68 diploid and 34 haploid chromosomes. The chromosome composition is apparently dimorphic, being constituted from two prominent V-shaped ones forming a homologous pair, and from 66 rod-shaped elements of varying lengths which are sorted into 32 homologous autosome pairs, and into one XY-pair of unequal size. The cytological evidences which seem to correlate with the taxonomical peculiarities were considered in the two forms under study.