males absent on the first

DEVELOPMENTAL BIOLOGY

Effects of Mutation or Deletion

In Drosophila, dosage compensation is controlled by the male-specific lethal (MSL) complex consisting of MSL proteins and roX RNAs. The MSL complex is specifically localized on the male X chromosome where it acts to increase its expression ~2-fold. A model for the targeted assembly of the MSL complex has been proposed in which initial binding occurs at ~35 dispersed chromatin entry sites, followed by spreading in cis into flanking regions. Here, one of the chromatin entry sites, the roX1 gene, was analyzed to determine which sequences are sufficient to recruit the MSL complex. Association and spreading of the MSL complex from roX1 transgenes was found in the absence of detectable roX1 RNA synthesis from the transgene. The recruitment activity was mapped to a 217 bp roX1 fragment that shows male-specific DNase hypersensitivity and can be preferentially cross-linked in vivo to the MSL complex. When inserted on autosomes, this small roX1 segment is sufficient to produce an ectopic chromatin entry site that can nucleate binding and spreading of the MSL complex hundreds of kilobases into neighboring regions (Kageymama, 2001).

MSL1 and MSL2 are thought to form a core complex that binds to chromatin entry sites, while MLE and other components including roX RNAs may be added sequentially. To assess whether all the MSL components are required to bind to the roX1 exons, MSL1 localization on the roX1c3.4 transgene was analyzed in msl mutant animals. Since msl mutant males show poor morphology of polytene chromosomes, females carrying a Hsp83-MSL2 transgene were analyzed. Females normally lack MSL2 and can not form MSL complexes, but when MSL2 is constitutively expressed in females, the complex forms and is localized to both X chromosomes. The roX1c3.4 transgene recruits the MSL complex in the absence of functional mof as well as in cells lacking msl3, as expected for an authentic chromatin entry site. Thus, functional msl3 and mof are not required for MSL binding to chromatin entry sites including roX1, but in their absence the complex fails to associate with the additional hundreds of sites seen in its wild-type pattern on the X chromosome. This has been interpreted as an inability of the complex to spread between chromatin entry sites in the absence of either MSL3 or MOF. To test this hypothesis at the level of an individual entry site, a strain was examined in which MSL1 showed frequent spreading from a roX1 transgene (roX1c3.4-51A). It was found that lack of functional msl3 or mof prevents spreading, as it does on the endogenous X chromosome. Taken together, the MLE helicase protein is essential for the MSL complex to bind to the roX1 transgene, while the MSL3 chromodomain protein and MOF histone acetyltransferase are required only for spreading (Kageymama, 2001).

Mutations in Drosophila ISWI, a member of the SWI2/SNF2 family of chromatin remodeling ATPases, alter the global architecture of the male X chromosome. The transcription of genes on this chromosome is increased 2-fold relative to females due to dosage compensation, a process involving the acetylation of histone H4 at lysine 16 (H4K16). Blocking H4K16 acetylation suppresses the X chromosome defects resulting from loss of ISWI function in males. In contrast, the forced acetylation of H4K16 in ISWI mutant females causes X chromosome defects indistinguishable from those seen in ISWI mutant males. Increased expression of MOF, the histone acetyltransferase that acetylates H4K16, strongly enhances phenotypes resulting from the partial loss of ISWI function. Peptide competition assays have revealed that H4K16 acetylation reduces the ability of ISWI to interact productively with its substrate. These findings suggest that H4K16 acetylation directly counteracts chromatin compaction mediated by the ISWI ATPase (Corona, 2002).

A functional dosage compensation complex required for male killing in Drosophila

Bacteria that selectively kill males ('male-killers') were first characterized more than 50 years ago in Drosophila and have proved to be common in insects. However, the mechanism by which sex specificity of virulence is achieved has remained unknown. This study tested the ability of Spiroplasma poulsonii to kill Drosophila melanogaster males carrying mutations in genes that encode the dosage compensation complex. The bacterium failed to kill males lacking any of the five protein components of the complex (Veneti, 2005).

Certain isofemale lines of Drosophila only give rise to daughters following the death of male embryos. Male death is due to the presence of intracellular bacteria that pass from a female to her progeny and that selectively kill males during embryogenesis. These male-killing bacteria are found in a wide range of other insect species, and many different bacteria have evolved male-killing phenotypes. In some host species, male-killers drive the host population sex ratio to levels as high as 100 females per male and alter the pattern of mate competition. However, the underlying processes that produce male-limited mortality are unclear. This study examined the interaction between the male-killing bacterium Spiroplasma poulsonii and the sex determination pathway of D. melanogaster (Veneti, 2005).

The primary signal of sex in Drosophila is the X-to-autosome ratio. This signal is permanently established in expression of Sex-lethal (Sxl) in females and its absence in males. This, in turn, effects three processes: germline sexual identity, somatic sexual differentiation, and dosage compensation, the process by which the gene expression titer on the X chromosome is equalized between two sexes despite their difference in X chromosome number. Mutations in the gene tra that convert XX individuals to male somatic sex do not induce female death. These observations indicate germline formation and migration happen correctly in male embryos and that dying male embryos do not express Sxl. Therefore, the requirement of the Spiroplasma for genes within the system of dosage compensation was examined (Veneti, 2005).

In Drosophila, the single X of males is hypertranscribed. This process of hypertranscription requires the formation of the dosage compensation complex (DCC) and its binding to (and modification of) the X chromosome. SXL in female Drosophila inhibits the production of MSL-2 protein, which is thus only present in male Drosophila. MSL-2 forms a complex with four other proteins, MSL-1, MSL-3, MLE, and MOF, which collectively form the DCC. MSL-1, MSL-3, MLE, and MOF are constitutively present in both males and females and are also supplied maternally. The complete DCC binds, with JIL-1, to the male X chromosome at various entry points, and, with the products of two noncoding RNAs, RoX1 and RoX2, it affects the modification of the single X chromosome and its hypertranscription (Veneti, 2005).

The effect was examined of mutations within the host DCC on the ability of the male-killer to function. The survival of male progeny beyond embryogenesis to L2/L3 (and in one case adult) was scored in the presence of different loss-of-function mutations within the dosage compensation system (normal male-killing occurs during embryogenesis), in the presence and absence of infection. Because many genes within this group additionally show strong maternal effects, the effect of mutations was in each case tested by using both mothers that were heterozygous for the loss-of-function mutation and mothers homozygous for it (Veneti, 2005).

Uninfected males homozygous for loss-of-function mutations within the dosage compensation system generally survive to the third larval instar. Survival to the third larval instar was studied for loss-of-function alleles of msl-1 (alleles msl-1¹ and msl-1^b), msl-2 (msl-2^g227 and msl-2^g134), msl-3 (msl-3¹³²), mle (mle⁹, mle¹), and mof (mof¹), and survival to adult for mle¹/mle⁶ transheterozygotes. In the case of all alleles of msl-1, msl-3, mle, and mof, a similar pattern is observed: Males homozygous or hemizygous for the loss-of-function mutation have appreciable survival in the presence of infection when their mother is also homozygous for the loss-of-function mutation. In contrast, heterozygous male embryos that were siblings of the above (that will have a wild-type DCC) were always killed, as were all male embryos in the case where the mother was heterozygous (where maternal supply of these proteins enables dosage compensation to be initiated, although not maintained). In the case of mle¹/mle⁶ transheterozygotes, male survival to adult was observed. For the case of mof, male-killing was restored to full efficiency when 18H1, a transgenic copy of mof, was added to the mof¹ loss-of-function background. Within the above crosses, three observations make sure the observation that Spiroplasma was fully operational. First, crosses involving heterozygous mothers, where male-killing was complete, were conducted concurrently with those using homozygous mothers, and the females in these crosses were siblings from the same vials. Second, in each cross and vial where homozygous males survived, heterozygous males (with wild-type function) still died. Finally, F₁ females derived from these crosses, when mated to wild-type males, produced a full, female-biased sex ratio (Veneti, 2005).

In the case of msl-2, where there is no maternal supply of MSL-2, survival of homozygous sons was observed for both homozygous and heterozygous mothers for the case of the mutation msl-2^g134. For the case of msl-2 ^g227, no male progeny were observed from infected females (table S5). This mutation does not rescue males in the assay, probably because the two mutations have different effects on msl-2 expression. The msl-2^g134 allele prevents formation of MSL-2 protein, whereas msl-2 ^g227 potentially encodes the RING-finger element of a truncated MSL-2 protein (Veneti, 2005).

Thus, absence or reduced function of any of the proteins of the DCC can reduce the efficiency of male killing, and a functional DCC is required for male killing by S. poulsonii. The fact that the genes mediating this process in Drosophila have been well studied can be exploited to yield further insights into the mechanism of male killing (Veneti, 2005).

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date revised: 20 January 2007

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