Evolution of the bilaterian germ line: lineage origin and modulation of specification mechanisms

A key focus of evolutionary developmental biology (evo–devo)in recent years has been to elucidate the evolution of developmentalmechanisms as a means of reconstructing the hypothetical lastcommon ancestors of various clades. Prominent among such reconstructionshave been proposals as to the nature of the mysterious "Urbilateria,"originally defined as the last common ancestor of the extantBilateria (protostomes and deuterostomes). Indeed, drawingsof this animal can now be found, as well as detailed informationon the genetics and morphological processes that it used toconstruct its gut, heart, eyes, appendages, segments, and bodyregions. Perhaps surprisingly, however, no explanations haveyet been offered as to how this animal might have achieved thesuccessful reproduction that must have been necessary for itto give rise to those lineages that are ancestral to today'sdiverse clades. The present article examines the comparativedata available to date on the specification of the only cellscontaining the genetic hereditary material, the germ cells,and speculates on the possible evolutionary and developmentalorigin of the Urbilaterian germ line.     

Patterns of molecular evolution of the germ line specification gene oskar suggest that a novel domain may contribute to functional divergence in Drosophila

In several metazoans including flies of the genus Drosophila, germ line specification occurs through the inheritance of maternally deposited cytoplasmic determinants, collectively called germ plasm. The novel insect gene oskar is at the top of the Drosophila germ line specification pathway, and also plays an important role in posterior patterning. A novel N-terminal domain of oskar (the Long Oskar domain) evolved in Drosophilids, but the role of this domain in oskar functional evolution is unknown. Trans-species transgenesis experiments have shown that oskar orthologs from different Drosophila species have functionally diverged, but the underlying selective pressures and molecular changes have not been investigated. As a first step toward understanding how Oskar function could have evolved, we applied molecular evolution analysis to oskar sequences from the completely sequenced genomes of 16 Drosophila species from the Sophophora subgenus, Drosophila virilis and Drosophila immigrans. We show that overall, this gene is subject to purifying selection, but that individual predicted structural and functional domains are subject to heterogeneous selection pressures. Specifically, two domains, the Drosophila-specific Long Osk domain and the region that interacts with the germ plasm protein Lasp, are evolving at a faster rate than other regions of oskar. Further, we provide evidence that positive selection may have acted on specific sites within these two domains on the D. virilis branch. Our domain-based analysis suggests that changes in the Long Osk and Lasp-binding domains are strong candidates for the molecular basis of functional divergence between the Oskar proteins of D. melanogaster and D. virilis. This molecular evolutionary analysis thus represents an important step towards understanding the role of an evolutionarily and developmentally critical gene in germ plasm evolution and assembly.     

Male germ cell specification and differentiation

Understanding the mechanisms by which the germline is induced and maintained should lead to a broader understanding of the means by which pluripotency is acquired and maintained. In this review, two major aspects of male germ cell development are discussed: underlying mechanisms for induction and maintenance of primordial germ cells and the basic signaling pathways that determine spermatogonial cell fate.     

PR结构域蛋白质与原始生殖细胞特化

原始生殖细胞特化在精子和卵子生成过程中发挥着重要的作用,而PR结构域蛋白质(PR-domain protein,PRDM)家族部分成员参与了该过程。PRDM1可抑制体细胞程序化过程中基因的表达,而PRDM1和PRDM14共同参与了潜在的全能性细胞的重新获取和基因组范围内表观遗传学重编程。这三个过程都是原始生殖细胞特化所必需的。此外,原始生殖细胞特化还需要一些其他因素如骨形态发生蛋白4(bone morphogenetic protein4,Bmp4)和RNA结合蛋白Lin28,这些因素通过影响PRDM发挥生理作用。对原始生殖细胞特化的理解有利于生殖细胞发育和相关问题的研究。     

The molecular machinery for lysosome biogenesis

The lysosome serves as a site for delivery of materials targeted for removal from the eukaryotic cell. The mechanisms underlying the biogenesis of this organelle are currently the subject of renewed interest due to advances in our understanding of the protein sorting machinery. Genetic model systems such as yeast and Drosophila have been instrumental in identifying both protein and lipid components of this machinery. Importantly, many of these components, as well as the processes in which they are involved, are proving conserved in mammals. Other recently identified components, however, appear to be unique to higher eukaryotes. BioEssays 23:333-343, 2001. Published 2001 John Wiley & Sons, Inc.     

The molecular machinery of resurgent sodium current revealed

Some TTX-sensitive sodium channels open transiently during recovery from inactivation, generating a "resurgent" sodium current that flows immediately following action potentials. In this issue of Neuron, Grieco and colleagues present evidence that resurgent sodium current results from a novel form of inactivation in which the cytoplasmic tail of the beta4 subunit acts as a classic open-channel blocker.     

The origin of Metazoa and Weismann's germ line theory

Weismann's theory asserts that the continuity of germ cells throughout the entire life cycle is insured by the protection of the somatic cells which represent a distinct lineage. On the basis of embryological data and the organization pattern displayed by some highly differentiated unicellular organisms, it is postulated that the soma of multicellular animals has probably arisen as an accessory structure for an initially unicellular system.     

Primordial germ cell specification: the importance of being 'blimped'

In mouse embryos, the expression of Blimp1 has recently revealed a population of allocated primordial germ cell precursors 24 hours earlier than previously thought. Those 'blimped' precursors have been shown to give rise, by mitotic division, to germ cells only and no other cell lineages. Here, we try to understand the events that lead to Blimp1 expression in the primordial germ cell precursors and speculate on what can be the role of Blimp1 during primordial germ cell specification and gastrulation in the mouse. Finally, we discuss the possible involvement of Blimp1 in the two know modes of germ line segregation (epigenesis and preformation).     

In vitro methods of male germ cell specification and differentiation

Germ line specification is an early cell fate decision essential for the transmission of totipotency over generations. Two types of germ line stem cells populate the male gonads in mammals. Primordial germ cells (PGCs) are the germ line founders only present during prenatal life. Spermatogonial stem cells (SSCs) appear a few days after birth and divide asymmetrically to give rise to one stem cell and one spermatogonia that initiates differentiation to produce spermatozoa. Germ cell specification and differentiation involve specific environmental stimuli and a sequential order of maturing phases required for gamete function. Spatio-temporal controls similarly dictate the erasure of somatic methylation marks and the subsequent acquisition of sex-specific marks at imprinted genes in gametes. We review here the recent advancements in male germ cell derivation from ES cells and discuss the limits of these in vitro methods in providing a kinetics and a microenvironment suitable for the programming of a proper gametic and parental identity.     

Plant male germ line transformation

A new method to produce transgenic plants - ma le ge rm li ne tr ansformation (MAGELITR) is reported. Unicellular tobacco microspores were isolated from excised anthers, and DNA carrying two marker genes was transferred biolistically. The bombarded microspores were matured in vitro for 6 days, and the mature pollen was used for in vivo pollination. Seeds were recovered and putative transformants were selected on the basis of their antibiotic resistance. Five kanamycin-resistant plants were chosen for further analysis, four contained the first construct, one the second. Parallel experiments with bicellular immature pollen did not produce any transgenic plants. A detailed DNA blot and expression analysis confirmed the transgenic nature of the five plants, and a genetic analysis showed that the transgenes are transmitted to subsequent generations. MAGELITR is a fast, regeneration-independent method, not prone to chimerism and somaclonal variation which should be genotype-independent and may be applicable in a wide range of species once in vitro maturation of pollen is established.     

The Dr-nanos gene is essential for germ cell specification in the planarian Dugesia ryukyuensis

Homologs of nanos are required for the formation and maintenance of germline stem cell (GSC) systems and for gametogenesis in many metazoans. Planarians can change their reproductive mode seasonally, alternating between asexual and sexual reproduction; they develop and maintain their somatic stem cells (SSCs) and GCSs from pluripotent stem cells known as neoblasts. We isolated a nanos homolog, Dr-nanos, from the expressed sequence tags (ESTs) of the sexualized form of Dugesia ryukyuensis. We examined the expression of Dr-nanos in asexual and sexualized planarians by in situ hybridization and analyzed its function using RNA interference (RNAi) together with a planarian sexualization assay. A nanos homolog, Dr-nanos, was identified in the planarian D. ryukyuensis. Dr-nanos expression was observed in the ovarian primordial cells of the asexual worms. This expression increased in proportion to sexualization and was localized in the early germline cells of the ovaries and testes. In X-ray-irradiated worms, the expression of Dr-nanos decreased to a large extent, indicating that Dr-nanos is expressed in some subpopulations of stem cells, especially in GSCs. During the sexualization process, worms in which Dr-nanos was knocked down by RNAi exhibited decreased numbers of oogonia in the ovaries and failed to develop testes, whereas the somatic sexual organs were not affected. We conclude that Dr-nanos is essential for the development of germ cells in the ovaries and testes and may have a function in the early stages of germ cell specification, but not in the development of somatic sexual organs.