Our Research

Overview

The embryonic gonad is composed of a few somatic cells intermingled with primordial germ cells (PGCs). During larval stages the embryonic gonad undergoes extensive changes  (Figure 2):

  • Significant proliferation of both somatic tissues and germ cells.
  • Somatic cell differentiation, and formation of the somatic niches for germ line stem cells (GSCs).
  • Establishment of a subgroup of PGCs as the future adult GSCs.

Thus, all the major steps that are required to transform a relatively undifferentiated organ to a well-constructed functioning organ occur during larval development.

 

embryogenesis
ovary
Figure 2A
An ovary at the end of embryogenesis. Germ cells (green) are embedded within the embryonic gonad (red, outlined). Very little patterning can be observed. Bar is 20 μm.
Figure 2B
An ovary at the end of larval development. Somatic niches for germ line stem cells have differentiated (Terminal filaments are separated by parallel lines, Cap cells are marked by arrowheads). Germ line stem cells are incorporated into somatic niches (outlined). A spherical fusome can be observed within GSCs. Germ cells that are not incorporated in the niches are beginning to differentiate and express the differentiation marker bam::GFP. Bar is 20 μm.
 

The processes that occur during larval gonadogenesis raise several questions:

  • What are the mechanisms that govern ovary growth, and how do somatic cells affect PGC proliferation?

  • How do the somatic cells of the ovary differentiate?

  • In particular, how does the somatic niche for germ line stem cells form?

  • How does the formation of the somatic niche affect the establishment of GSCs?

The lab concentrates on two major fields of study:

intermingled cells
Figure 3
ICs and PGCs are in direct contact. At the end of larval development, PGCs (green) occupy the middle portion of the gonad. Somatic cells are outlined (blue). PGCs are associated with a specific cell population, Intermingled cells (ICs, red).
PGC proliferation

Figure 4
  1. Control of proliferation in the larval ovary

    Our long-term objectives are to understand how the many pathways that control PGC proliferation connect into a network within the growing organ. Such a network will combine physiological inputs and cues from somatic neighbors that must be integrated in PGCs to affect their cell cycle.

    We have shown that a specific somatic cell population (Intermingled Cells, ICs) (Figure 3) coordinates its growth with PGCs through an Epidermal Growth Factor Receptor (EGFR)-mediated feedback loop (Gilboa and Lehmann, 2006). PGCs produce Spitz, an EGFR ligand, which is required for IC survival. EGFR signaling is also responsible for production of a substance emanating from ICs and repressing PGC proliferation (Figure 4).

      1. Using both microarray and candidate gene approaches, we are now trying to uncover the identity of this repressive signal.
      2. In a genetic screen we uncovered novel regulators of germ cell proliferation. Many of them are homologues of known human oncogenes. We will study the developmental role of these genes and how their over-expression leads to aberrant germ cell proliferation. We are mostly interested in how somatic cells control PGC proliferation
  1. Adult stem cell establishment

    The Drosophila ovary is one of the most influential models for how stem cell maintenance and differentiation is affected by the niche. Despite this, we know of relatively few genes that are affecting stem cell biology. Moreover, many of those genes were discovered by a candidate gene approach, rather than by genetic screens.
    To find new genes that are affecting stem cell biology, we performed an over-expression screen, searching for stem cell defects within the larval ovary. The screen was based on our previous work, which showed that the same pathways responsible for stem cell maintenance in the adult, are acting in the larval ovary to repress PGC differentiation (Gilboa and Lehmann, 2004). We reasoned that in larval ovaries, where wild-type PGC never differentiate, any phenotype of precocious PGC differentiation will be immediately recognized (Figure 5).
    The screen uncovered both autonomous and non-autonomous regulators of stem cell maintenance (Figure 5). Some were previously uncovered by candidate gene approaches and some are novel. The study of these genes will provide us with a better understanding of how adult stem cells function in-vivo.
     

    figure 5
    figure 5
    figure 5
    Figure 5
    Screen for stem cell maintenance defects. (WT) In wild-type larval ovaries, PGCs divide and separate to form two PGCs. Each PGC carries a spherical fusome (arrows). Occasionally fusomes with dumbbell shapes (arrowhead), indicating two daughter cells still connected, can be observed. (A-B) Genes that interfere with normal stem cell maintenance also cause precocious PGC differentiation within the larval ovary. Extended fusomes, which indicate the formation of differentiating germ line cysts within the larval ovary, can be observed (arrowheads).