Germline Research

Investigation of Inductive Mechanisms in Arthropod Germ Line Specification
Ben Ewen-Campen and Evelyn Schwager
In the fruit fly Drosophila melanogaster, primordial germ cells (PGCs) are specified by inherited cytoplasmic germ line determinants, while in some vertebrate systems, inductive BMP 2/4 signals determine PGC fate. In most arthropods, PGCs are similarly first observed late in embryogenesis, suggestive of induction of PGC fate through signaling pathways, and in contrast to the relatively derived fruit fly model. The expression of arthropod BMP 2/4 homologues (dpp genes) suggests that the Dpp signalling pathway may be an ancestral component of PGC specification in both arthropods and bilaterians. This hypothesis will be tested using two arthropod models: the cricket Gryllus bimaculatus, and the spider Parasteatoda tepidariorum.

Diversification of Germ Cell Migration Mechanisms
Friedemann Linsler
Germ cells normally undertake long and complex migration routes through the forming embryo in order to finally reach the somatic component of the gonads. Successful completion of this route is pivotal for reproduction and hence survival of the species. Germ cell migration is a classical topic of developmental biology and has been subject to intense investigation in model organisms like fruit flies and mice. Serving as a paradigm for cell migration, it is relevant for many kinds of processes like organ development, immune system function, wound healing or carcinogenesis, which also directly employ several of the genes known to function in PGC migration.  Years of study by several researchers have revealed conservation in the employment of genes not only between different cellular migration processes, but also between different species.  The phylogenetic spread of the well studied model organisms, together with the several highly conserved genes employed in the germline, suggest that germ cell migration processes should reveal important clues about the embryonic development and reproductive system of the enigmatic ancestor of all bilaterian animals, which lived 550 to 800 million years ago. We are studying the cellular behaviour and genetic mechanisms involved in migration of the germ cells in the new model organism Parhyale hawaiensis.

The Effect of Ovarian Architecture on Embryonic Patterning
(Ben Ewen-Campen)

We would like to know to what extent the mechanisms that localise determinants for the germ line and axial patterning are conserved across the insects, and affected by ovarian architecture.  Three types of ovaries are found among the insects.  Two of these types have nurse cells, and are called meroistic; depending on the arrangement of the nurse cells with respect to the oocyte, meroistic ovaries are either polytrophic or telotrophic.  The third type has no nurse cells, and is called panoistic.

Drosophila has a polytrophic meroistic ovary, where each oocyte is accompanied by and connected to 15 polyploid nurse cells, which are also of germ line origin. This ovarian structure is common among some of the holometabolous insects, and is considered the most derived type of insect ovary. The localisation of germ line and axial patterning determinants proceeds autonomously in each oocyte. The nurse cells produce mRNAs and proteins that are carried through cytoplasmic bridges to the oocyte, where they are localised to specific ooplasmic domains.

In contrast, in the telotrophic meroistic ovaries of the milkweed bug Oncopeltus fasciatus, all oocytes share contact with a common pool of nurse cells, which are contained in an anterior compartment called the tropharium.  Oocytes are connected by to the tropharium by tubes called trophic cords.  The organisation of this ovary, which is characteristic of many holometabolous insects, raises questions about the mechanisms that localise determinants within the oocytes.  Microtubule-dependent motor proteins are likely to be responsible for at least some aspects of transport in this system.

In panoistic ovaries, there are no nurse cells at all.  However, insects that have panoistic ovaries also produce polarised oocytes, and lay polarised eggs.  Gryllus bimaculatus is our model for ovaries of this type.  We are examining which germ line and axial patterning determinants, in the form of specific mRNAs and/or proteins, might be produced and transported in telotrophic meroistic and panoistic ovaries, and whether these mRNAs are specifically involved in embryonic patterning and development.