Reduced cul-5 activity causes aberrant follicular morphogenesis and germ cell loss in Drosophila oogenesis

Abstract

Drosophila oogenesis is especially well suited for studying stem cell biology, cellular differentiation, and morphogenesis. The small modifier protein ubiquitin regulates many cellular pathways. Ubiquitin is conjugated to target proteins by a diverse class of enzymes called ubiquitin E3 ligases. Here we characterize the requirement of Cul-5, a key component of a subgroup of Cullin-RING-type ubiquitin E3 ligases, in Drosophila oogenesis. We find that reduced cul-5 activity causes the formation of aberrant follicles that are characterized by excess germ cells. We show that germ line cells overproliferate in cul-5 mutant females, causing the formation of abnormally large germ line cysts. Also, the follicular epithelium that normally encapsulates single germ line cysts develops aberrantly in cul-5 mutant, leading to defects in cyst formation. We additionally found that Cul-5 is required for germ cell maintenance, as germ cells are depleted in a substantial fraction of cul-5 mutant ovaries. All of these cul-5 phenotypes are strongly enhanced by reduced activity of gustavus (gus), which encodes a substrate receptor of Cul-5-based ubiquitin E3 ligases. Taken together, our results implicate Cul-5/Gus ubiquitin E3 ligases in ovarian tissue morphogenesis, germ cell proliferation and maintenance of the ovarian germ cell population.

Figure 1. cul-5 mutant ovarioles form aberrant egg chambers.

Ovarioles from control (A) and cul-5^EY21463 mutant (B–L) females, labelled for Vas (white) and DNA (blue). (A, B) Control (A) and many cul-5 mutant (B) ovarioles show normal morphology. In a substantial fraction of cul-5 mutant ovarioles (C–J), one or several egg chambers contain more than 16 germ line cells (aberrant egg chambers). (C, D) Cysts in region III of cul-5 mutant germaria are often irregularly shaped and fail to take up a oval or round shape as wild type cysts do. (E) Frequently, several follicles containing excess germ cells are observed in a single ovariole. (F) More rarely, cul-5 mutant egg chambers contain less than 16 germ line cells. (G) More mature aberrant egg chambers can develop relatively normally. (H) Aberrant egg chambers are also observed in a different allelic combination. (I) In some cul-5 mutant ovarioles, individual egg chambers are not separated by a stalk, but by two layers of follicular epithelium. (J) A fraction of cul-5 ovarioles has germaria that do not contain germ cells. (K, L) In aged flies, a fraction of ovarioles undergoes morphogenetic catastrophe and lose the highly ordered structure of normally developing ovarioles. See Table 1 for quantification of this phenotype.

Figure 2. Germ cells in aberrant egg chambers differentiate into oocytes and nurse cells.

Ovarioles and egg chambers from wild type (A) and cul-5 ^EY21463 mutant (B–H) females. (A, B) Wild type and many cul-5 mutant egg chambers accumulate the oocyte marker Orb (white) in a single cell in the posterior of the follicle. (C–E) While some aberrant egg chambers containing more than 16 germ cells contain only a single Orb-positive cell (white; DNA labelled blue), others contain two (D) or more (E) Orb-positive cells. (F) Follicles with fewer than 16 germ line cells may also contain an Orb-positive cell. (G) Ovarioles that have undergone morphogenetic catastrophe still contain Orb-positive cells of variable size (Orb is labelled green, Vas in white). (H) The oocyte marker Bic-D accumulates in a pattern very similar to Orb in cul-5 mutant egg chambers.

Figure 3. Germ line clusters overproliferate in the germarium of cul-5 mutant ovarioles.

(A–E) Wild type (A) and cul-5 ^EY21463 mutant ovarioles (B–E) were stained for Vas (blue) and phospho-histone H3 (white). (A) In wild type ovarioles, small numbers of germ cells divide synchronously in the germarium. (B–E) In some cul-5 mutant ovarioles, large numbers of germ line cells undergo mitosis in a germarium, while we never observe germ cell divisions in the distal germarium or in the vitellarium (B). (D–E) are projections of a Z-series taken through (C). The asterisk indicates dividing cells of somatic origin in the projections. (F) In wild type egg chambers, each oocyte (arrow) contains exactly four ring canals enriched in F-actin (labelled with Phalloidin). (G) In aberrant egg chambers in cul-5 mutants, more than four ring canals are observed in oocytes (arrow) and nurse cells. (H–K) cul-5 mutant germaria (J–K) feature large fusomes (labelled with mAb 1b1) spanning more than 16 cyst cells that are never observed in wild type (H–I) germaria. (H, J) are single 0.5 µm planes, while (I, K) are 3D reconstructions of corresponding Z-stacks spanning the width of the germaria. The bracket in K indicates a single polyfusome that is traceable through the entire Z-stack.

Figure 4. cul-5 mutant follicles do not contain 2ⁿ germ line cells.

The graph plots the number of germ cells (Y-axis) in 20 randomly chosen aberrant follicles (X-axis). The green and red lines indicate the normal 16 (green) germ cells per follicles, or multiples of 16 (red). For each follicle, we counted the number of germ line cells three times (error bars denote ±SD). The number at the bottom of the column indicates the number of Orb-positive cells (oocytes) found in that particular follicle.

Figure 5. Follicular morphogenesis in aberrant cul-5 mutant egg chambers.

(A–H) Wild type (A–B) and cul-5 ^EY21463 mutant (C–H) egg chambers were labelled for Vas (red) and FasIII (A, C–F) or EyA (B, G–H) (green in A–C, E, G or white in D, F, H). (A–C, E, G) are single confocal planes, while (D, F, H) are projections of Z-stacks spanning the entire corresponding egg chamber. (A) In wild type, FasIII accumulates in two pairs of polar cells at the anterior and posterior pole of the follicle (arrows). (B) EyA accumulates in a complementary pattern, and is excluded from the anterior and posterior polar cells while it accumulates in the nuclei of all other follicle cells. (C–D) In most aberrant egg chambers in cul-5 mutants, only the normal two groups of polar cells are specified. (E–H) Only in small fraction of aberrant cul-5 follicles, more than two groups of cells have upregulated FasIII (E–F) or downregulated EyA (G–H).