Germline Chromosomes Research Articles

Mite chromosomes: an exceptionally small number.

ALTHOUGH the chromosomes of thousands of plants and animals have been investigated and chromosome numbers found to vary widely, the overall trend has been toward the evolution of a moderate number in most taxa. Very few species have been reported to have as few as n = 2 chromosomes. Jackson1 reported the flowering plant Haplopappus gracilis to be n = 2 and most cytology textbooks indicate that the nematode Parascaris equorum (= Ascaris megalocephala) possesses one pair of chromosomes in germ line cells. In the latter species, however, the two chromosomes fragment into numerous small chromosomes in the soma, and therefore the germ line chromosomes are considered aggregate bodies2. In the Insecta some coccid members of the tribe Iceryini have a haploid number of two chromosomes and unfertilized haploid eggs develop parthenogenically into males3. Sokolov4 found two species of water mites (Eylais rimosa and Eylais setosa) to be n = 2, but these species do not display haploid parthenogenesis. The rarity of reported cases of such a low chromosome number throughout the plant and animal kingdoms prompted us to report the following data.

The elimination and differentiation of chromosomes in the germ line of sciara.

The germ line chromosomes of S. coprophila have been followed from the time of origin of the germ cells up to the time of meiosis in the male and up to first larval molt in the female. The mechanism which prevents the accumulation of L (limited) chromosomes in the germ line is a unique process of chromosome elimination: it occurs in male and female embryos after the germ cells have migrated from the pole plasm to the definitive gonad site, and it involves the movement of whole L chromosomes through the nuclear membrane into the cytoplasm. The extra paternal X chromosome is eliminated from the germ cells at the same time and in the same manner. Following this elimination there is a cytological differentiation of the chromosomes remaining inside the nucleus. First, the 4 paternal homologues of the regular complement undergo a loosening of coils and become light-staining whereas the maternal homologues remain condensed like the L's. Next, the L chromosomes undergo a process of extreme attenuation and dispersion following which they return to the condensed state. H3-thymidine autoradiography on gonial and premeiotic cells in the testis reveals that the L chromosomes undergo DNA replication at the end of the S period, also that there are asynchronies in DNA synthesis among the regular chromosomes. The phenomena of differential chromosome staining and asynchronous DNA replication are considered in the light of current theory regarding heterochromatization and gene inactivation, also in relation to the phenomenon of chromosome imprinting encountered in this genus.

The Nature of Lampbrush Chromosomes

This chapter discusses the nature of lampbrush chromosomes, which are germ-line chromosomes with laterally projecting loops. The centromeres of newt lampbrush chromosomes are Feulgen-positive granules without lateral loops, whereas most other chromomeres bear loops. In oocyte nuclei, the lateral loops are metabolically active, whereas in all probability the centromeres are passive. During mitosis, the centromeres are assuredly active, but even in the large mitotic chromosomes of newts they are only visible as negatives—that is, as unstained gaps in otherwise uniformly Feulgen-positive cylinders. The chiasmata that associate homologous lampbrush chromosomes are connections established between chromosomes' axes. This may indicate that genetic recombination is restricted to inter-chromomeric regions, though the compact chromomeric type of organization of lampbrush chromosomes could conceal intra-chromomere strand exchanges. The chapter discusses the polarity of lateral loops, the hypothesis of polarized extension and retraction of lampbrush chromosomes, and the genetic sources of morphological variation of lampbrush chromosomes.

THE MALE SOMATIC CHROMOSOME GROUP IN SCIARA PAUCISETA

The present study is designed to furnish additional evidence bearing on the general problem of sex determination in Sciara. It is concerned particularly with two features. In the first place, evidence is given showing that in Sciara pauciseta Felt there are seven chromosomes in the male soma instead of eight as in the female soma, although the germ-line chromosome groups are alike in the two sexes. In the second place, it is shown that this evidence adds support to the hypothesis that the precocious 2 chromosome in Sciara is a sex chromosome (Metz, 1929, 1930). Earlier evidence bearing on the immediate problem may be summarized briefly as follows: 1. In all species of Sciara thus far adequately studied, the chiromosome group of the germ-line in both sexes (i.e., spermatogonia and oogonia) consists of symmetrical pairs of chromosomes-except for variations in the number of limited chromosomes which are found