We performed threefold shotgun sequencing of the silkworm (Bombyx mori) genome to obtain a draft sequence and establish a basic resource for comprehensive genome analysis. By using the newly developed RAMEN assembler, the sequence data derived from whole-genome shotgun (WGS) sequencing were assembled into 49,345 scaffolds that span a total length of 514 Mb including gaps and 387 Mb without gaps. Because the genome size of the silkworm is estimated to be 530 Mb, almost 97% of the genome has been organized in scaffolds, of which 75% has been sequenced. By carrying out a BLAST search for 50 characteristic Bombyx genes and 11,202 non-redundant expressed sequence tags (ESTs) in a Bombyx EST database against the WGS sequence data, we evaluated the validity of the sequence for elucidating the majority of silkworm genes. Analysis of the WGS data revealed that the silkworm genome contains many repetitive sequences with an average length of <500 bp. These repetitive sequences appear to have been derived from truncated transposons, which are interspersed at 2.5- to 3-kb intervals throughout the genome. This pattern suggests that silkworm may have an active mechanism that promotes removal of transposons from the genome. We also found evidence for insertions of mitochondrial DNA fragments at 9 sites. A search for Bombyx orthologs to Drosophila genes controlling sex determination in the WGS data revealed 11 Bombyx genes and suggested that the sex-determining systems differ profoundly between the two species.

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The genome sequence of silkworm, Bombyx mori

Centrioles are present in oogonia and oocytes of hamsters, mice, rats, gerbils and man up to the pachytene stage. In subsequent meiotic stages, centrioles are absent. At the time of germinal vesicle breakdown in the mouse several foci composed of electron-dense fibrillar material from which microtubules radiate appear near the nucleus. These fibrillar aggregates, referred to as microtubule foci, form small asters and collectively perform the role of the spindle pole during the two meiotic divisions. There are multiple centres at each pole and their number may vary. Thus, centrioles are not obligatory organelles of the spindle apparatus.

Absence of Centrioles in the First and Second Meiotic Spindles of Mouse Oocytes

Publisher Summary Advances in the understanding of the formation, structure, and chemistry of the mammalian kinetochore and its associated fiber are summarized in the chapter. The initiation of chromosome movement during cell division can be correlated with the formation of a fiber, composed primarily of microtubules (MTs) and associated proteins that connect each chromosome to the polar area of the spindle. The region on the chromosome where the MTs attach is the kinetochore, and the MTs themselves are known as “kinetochore MTs” (K-MTs). The origin of K-MTs is consistent with the morphological changes that occur within the astral spindle during prometaphase, with the structure of the K-fiber, with the ultrastructural data on prometaphase in many types of cells, with the in vivo nucleation data, and with the in vivo polarity determinations. The origin of K-MTs offers an explanation for various in vivo observations that have yet to be explained by a mechanism of K-MT formation, based solely on the nucleation of MTs by the kinetochore.

The formation, structure, and composition of the mammalian kinetochore and kinetochore fiber.

The thorough descriptive studies by the early cytologists Meves (1901, 1907), Retzius (1904, 1909), Bowen (1920, 1922 a-c, 1924), Pollister (1930), and Johnson (1931) provided later workers with a remarkably accurate picture of the microscopic anatomy of insect spermatozoa and of spermiogenesis in several insect species . Their light microscope observations were necessarily lacking in detail, however, and since the advent of the electron microscope, an increasing number of cell biologists have undertaken to extend our understanding of the structure of insect germ cells. Some have confined their attention to spermiogenesis or the morphology of mature spermatozoa of a particular species, while others have been concerned with the comparative morphology of single sperm organelles. Although there are by now many relevant papers of limited scope widely dispersed in the biological literature, there seems to be no single reference to which one can turn for an overview of the structure and development of the insect spermatozoon. In this review an effort is made to gain some perspective from a synthesis of these fragmentary accounts supplemented by the author's own observations based on electron microscopic examination of nearly two hundred insect species.'

/pdf/insect-sperm-their-structure-and-morphogenesis-clidb2sb19.pdf

Insect sperm: their structure and morphogenesis.

Publisher Summary Mitosis is defined as all those types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus. The greatest range of variations by which mitosis is accomplished, occurs in the protistan and fungal kingdoms, some members of which are probably most similar to the ancestors of higher plants and animals. The variations in the higher organisms are secondarily derived from the division patterns of typical plants and animals. This chapter discusses the characteristics and evolution of mitosis. The efficiency of mitosis consists of two basic components; the frequency with which each daughter nucleus receives the necessary complete genome complement (genetic efficiency) and the amount of energy and materials expended in the synthesis and operation of the mitotic apparatus. The chapter also discusses the use of mitosis as a phylogenetic marker. It is applicable to all eukaryotic cells and thus is valuable across boundaries where other structures are absent on one side and present in various forms on the other side.

Variant mitoses in lower eukaryotes: indicators of the evolution of mitosis.

The spindle of metazoan cells functions as a dual distributor that guarantees the accurate segregation of both chromosomes and centrioles (basal bodies). This combined mechanism may have evolved from the separate distribution devices for chromosomes and basal bodies found in Protozoa. Typical eupyrene spermatocytes of the silk moth were compared with atypical apyrene spermatocytes. ( a ) Long microtubules, persisting during centriolar movements, develop in the meiotic prophases in continuity with the centrioles ( b ) There is no longer a centriole-microtubule continuity at metaphase-anaphase ( c ) Spindles comprise microtubules and endoplasmic reticulum (ER). The microtubules form a barrel-shaped structure terminating far in front of the centrioles. The two types of spermatocytes differ in the structure of the ER, which is vesicular in the eupyrene and lamellar in the apyrene line. It is suggested that the ER influences the anaphase movement of chromosomes, ( d ) The chromosomes lack localized centromeres. Microtubules penetrate all along the polar faces of the eupyrene metaphase chromosomes. The apyrene chromosomes form irregular blocks that are penetrated by microtubules all around their periphery, ( e ) The centriole comprises 3 concentric zones. Typical changes in the centriole are correlated with changes in the cell cycle. Replication, de novo formation and disappearance of centrioles are manifestations of the state of flux of the microtubules.

The spindle as a basal body distributor. A study in the meiosis of the male silkworm moth, Bombyx mori.

Centrioles were studied in living and fixed testes of neuropteran insects by light, phase-contrast, and electron microscopy. Giant centrioles, measuring up to 8.0 µ in length (according to the species), develop in meiotic prophase, whereas only minute centrioles are found in spermatogonia. The giant centrioles have essentially the same ultrastructure as described for smaller centrioles, but their periodic structure is more clearly defined. The growth of giant centrioles consists of two phases. First, a procentriole lengthens to the size of its mother centriole, as in mitosis; then the four centriolar rods of one cell continue their growth simultaneously and at the same rate. At their distal ends centriolar vesicles are attached. These originate from invaginations of the cell membrane and apparently contribute to the growth of the centrioles. Neuropteran spermatids, like those of other insects, have only one centriolar rod, while spermatids of many other animals contain two centrioles. The origin and significance of this difference are unknown.

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J. Wahrman

Papers

The spindle as a basal body distributor. A study in the meiosis of the male silkworm moth, Bombyx mori.

Giant centrioles in neuropteran meiosis.