Maternal-offspring conflict leads to the evolution of dominant zygotic sex determination

Sex determination in many species involves interactions among maternally expressed genes (eg, mRNA's and proteins placed into the egg) and zygotically expressed genes. Recent studies have proposed that conflicting selective pressures can occur between maternally and zygotically expressed sex determining loci and that these may play a role in shaping the evolution of sex determining systems. Here we show that such genetic conflict occurs under very general circumstances. Whenever sex ratio among progeny in a family affects the fitness of either progeny in that family or maternal fitness, then maternal-zygotic genetic conflict occurs. Furthermore, we show that this conflict typically results in a "positive feedback loop" that leads to the evolution of a dominant zygotic sex determining locus. When males more negatively effect fitness within the family, a male heterogametic (XY male) sex determining system evolves, whereas when females more negatively effect fitness in the family, a female heterogametic (ZW female) system evolves. Individuals with the dominant sex allele are one sex, and the opposite sex is determined by maternally-expressed genes in individuals without the dominant sex allele. Results therefore suggest that maternal-zygotic conflict could play a role in the early evolution of chromosomal sex determining systems. Predictions are made concerning the patterns of expression of maternal and zygotic sex determining genes expected to result from conflict over sex determination.     

A major role for zygotic hunchback in patterning the Nasonia embryo

Developmental genetic analysis has shown that embryos of the parasitoid wasp Nasonia vitripennis depend more on zygotic gene products to direct axial patterning than do Drosophila embryos. In Drosophila, anterior axial patterning is largely established by bicoid, a rapidly evolving maternal-effect gene, working with hunchback, which is expressed both maternally and zygotically. Here, we focus on a comparative analysis of Nasonia hunchback function and expression. We find that a lesion in Nasonia hunchback is responsible for the severe zygotic headless mutant phenotype, in which most head structures and the thorax are deleted, as are the three most posterior abdominal segments. This defines a major role for zygotic Nasonia hunchback in anterior patterning, more extensive than the functions described for hunchback in Drosophila or Tribolium. Despite the major zygotic role of Nasonia hunchback, we find that it is strongly expressed maternally, as well as zygotically. Nasonia Hunchback embryonic expression appears to be generally conserved; however, the mRNA expression differs from that of Drosophila hunchback in the early blastoderm. We also find that the maternal hunchback message decays at an earlier developmental stage in Nasonia than in Drosophila, which could reduce the relative influence of maternal products in Nasonia embryos. Finally, we extend the comparisons of Nasonia and Drosophila hunchback mutant phenotypes, and propose that the more severe Nasonia hunchback mutant phenotype may be a consequence of differences in functionally overlapping regulatory circuitry.     

Maternal-effect genes that alter the fate map of the Drosophila blastoderm embryo

The pattern of segmentation in the Drosophila embryo is controlled by at least 25 zygotically active genes and at least 20 maternally active genes. We have examined the pattern of expression of the protein product of the zygotically active segmentation gene fushi tarazu (ftz) at the cellular blastoderm stage in progeny of mutant females homozygous for each of six maternal-effect segmentation genes to observe the early effects of the maternal-effect genes on zygotic gene expression. The genes included exuperantia (a member of the anterior class of maternal-effect segmentation genes); staufen and vasa (members of the posterior class); and torso, trunk, and fs(1)N (members of the terminal class). Mutations in the genes caused a disruption of the normal pattern of ftz stripes in regions of the embryo where gene activity is known to be required. The ftz stripes provide a marker for segmental determination at the cellular blastoderm stage, making it possible to correlate aberrant patterns of ftz protein with defects in cuticle morphology at the end of embryogenesis. ftz protein expression in progeny of females mutant for combinations of the above genes was also examined. The changes in the ftz pattern in progeny of females doubly mutant for genes of the anterior and terminal classes or of the posterior and terminal classes can largely be understood as the result of the additive effects of the single mutations. In contrast, clearly nonadditive effects on the ftz pattern were seen when a mutation in a gene of the anterior class (exuperantia) was combined with mutations in posterior class genes.     

Developmental analysis of the torso-like phenotype in Drosophila produced by a maternal-effect locus

The segmental plan of the Drosophila embryo is already established at the blastoderm stage through the action of maternal effect genes which determine the polarity of the embryo and zygotically active genes involved in segmentation. We have analyzed the first example of a group of maternally acting genes which are necessary for establishing the developmental potential of the posterior 25% of the blastoderm. Females, homozygous for the X-linked maternal-effect mutation female sterile(1)Nasrat211 [fs(1)N211], produce embryos, characterized as torso-like, which lack all posterior endodermal derivatives as well as structures characteristic of abdominal segments 8 to 10. In addition, anterior endodermal derivatives are deficient and the absence of pharyngeal musculature causes a collapse of the cephalopharyngeal apparatus. The columnar blastoderm cell layer is defective at the posterior tip below the pole cells in these embryos. This defect, however, is presumably secondary to some abnormal feature of pole cell formation since in double mutants of fs(1)Nasrat211; tudor3 the blastoderm is normal but the embryos still show the torso-like phenotype. In situ hybridization with RNA probes derived from the fushi tarazu gene establishes that the cellular determination of the posterior blastoderm of embryos produced by fs(1)N211 is changed. This represents the first direct demonstration that a maternal-effect mutation alters the spatial distribution of a zygotic gene product involved in the segmental patterning of the embryo.     

Maternal effects and temperature-sensitive period of mutations affecting embryogenesis in Caenorhabditis elegans

We have used standard tests to investigate the nature of gene expression of a new set of temperature-sensitive mutants defining 30 emb genes (essential for embryogenesis) in the nematode Caenorhabditis elegans. The mode of gene expression as determined by progeny tests for parental effects divides the genes into four classes. For 18 genes maternal gene expression is necessary and sufficient for normal embryogenesis; for 2 genes zygotic expression is necessary and sufficient; for 7 genes either maternal or zygotic expression is sufficient; for 3 genes both maternal and zygotic expression are necessary. One mutant displayed partial paternal sufficiency. The results of temperature-shift experiments define two “execution stages,” corresponding to the limits of the temperature-sensitive period (TSP), and indicate the nature and the time of action or synthesis of the gene products. Most of the maternally expressed genes have very early execution stages indicating translation before fertilization, but some are temperature sensitive late in embryogenesis. Early execution stages for 2 zygotically necessary genes demonstrate that the zygotic genome can be active in the earliest stages of embryogenesis. All taken together, the mode of gene expression, TSP, and arrest stage (terminal phenotype) allow us to classify functionally and begin to order the genes essential for embryogenesis. The results indicate a preeminent role for maternal genes and gene products in embryogenesis, in agreement with the results of others.     

Maternal-Zygotic Lethal Interactions in DROSOPHILA MELANOGASTER: The Effects of Deficiencies in the Zeste-White Region of the X Chromosome

The possibility that essential loci in the zeste-white region of the Drosophila melanogaster X chromosome are expressed both maternally and zygotically has been tested. Maternal gene activity was varied by altering gene dose, and zygotic gene activity was manipulated by use of position-effect variegation of a duplication. Viability is affected when both maternal and zygotic gene activity are reduced, but not when either maternal or zygotic gene activity is normal. Tests of a set of overlapping deficiencies demonstrate that at least three sections of the zeste-white region yield maternal zygotic lethal interactions. Single-cistron mutations at two loci in one of these segments have been tested, and maternal heterozygosity for mutations at both loci give lethal responses of mutant-duplication zygotes. Thus, at least four of the 13 essential functions coded in the zeste-white region are active both maternally and zygotically, suggesting that a substantial fraction of the genome may function at both stages. The normal survival of zygotes when either maternal gene expression or zygotic gene expression is normal, and their inviability when both are depressed, suggests that a developmental stage exists when maternally determined functions and zygotically coded functions are both in use.     

The Thick Veins Gene of Drosophila Is Required for Dorsoventral Polarity of the Embryo

We have discovered a new member of the class of genes controlling embryonic dorsoventral patterning. Mutants of the thick veins (tkv) gene have been described previously (as slater alleles) as embryonic lethal, lacking dorsal epidermis, but not as showing a recognizable dorsoventral phenotype. We show here that maternal alteration of function coupled with zygotic reduction of function of tkv is strongly ventralizing. In addition, in double heterozygous combinations in the mother, tkv mutations increase the ventralizing effect of dominant, weakly ventralizing alleles of the maternal effect, dorsoventral genes easter and cactus. An interaction is also seen with zygotic dorsoventral genes: tkv interacts maternally and zygotically in double heterozygotes with decapentaplegic and zygotically with screw in double homozygotes. We conclude that both maternally and zygotically supplied wild-type tkv product can play a role in dorsoventral patterning of the early embryo. On the basis of the phenotype of trans-heterozygous adult escapers, we propose that tkv might act by potentiating the activity of the zygotically acting decapentaplegic gene.     

Developmental use of gene products in Drosophila: the maternal-zygotic transition

Recent results suggest that activity of a large fraction of the Drosophila genome is needed at multiple developmental stages. The timing of the transition from dependence on maternally stored gene products to reliance on zygotically coded products has been examined for several zygotic-lethal mutations in the z-w region of the X chromosome. The mutants differ in zygotic sensitivity to reduced maternal activity, and they have a wide range of times of lethality. Nevertheless, both temperature shift experiments and clonal analysis indicate that all of the maternal-zygotic transitions occur around the time of blastoderm formation.     

Maternal expression relaxes constraint on innovation of the anterior determinant, bicoid

The origin of evolutionary novelty is believed to involve both positive selection and relaxed developmental constraint. In flies, the redesign of anterior patterning during embryogenesis is a major developmental innovation and the rapidly evolving Hox gene, bicoid (bcd), plays a critical role. We report evidence for relaxation of selective constraint acting on bicoid as a result of its maternal pattern of gene expression. Evolutionary theory predicts 2-fold greater sequence diversity for maternal effect genes than for zygotically expressed genes, because natural selection is only half as effective acting on autosomal genes expressed in one sex as it is on genes expressed in both sexes. We sample an individual from ten populations of Drosophila melanogaster and nine populations of D. simulans for polymorphism in the tandem gene duplicates bcd, which is maternally expressed, and zerknüllt (zen), which is zygotically expressed. In both species, we find the ratio of bcd to zen nucleotide diversity to be two or more in the coding regions but one in the noncoding regions, providing the first quantitative support for the theoretical prediction of relaxed selective constraint on maternal-effect genes resulting from sex-limited expression. Our results suggest that the accelerated rate of evolution observed for bcd is owing, at least partly, to variation generated by relaxed selective constraint.     

An F1 genetic screen for maternal-effect mutations affecting embryonic pattern formation in Drosophila melanogaster

Large-scale screens for female-sterile mutations have revealed genes required maternally for establishment of the body axes in the Drosophila embryo. Although it is likely that the majority of components involved in axis formation have been identified by this approach, certain genes have escaped detection. This may be due to (1) incomplete saturation of the screens for female-sterile mutations and (2) genes with essential functions in zygotic development that mutate to lethality, precluding their identification as female-sterile mutations. To overcome these limitations, we performed a genetic mosaic screen aimed at identifying new maternal genes required for early embryonic patterning, including zygotically required ones. Using the Flp-FRT technique and a visible germline clone marker, we developed a system that allows efficient screening for maternal-effect phenotypes after only one generation of breeding, rather than after the three generations required for classic female-sterile screens. We identified 232 mutants showing various defects in embryonic pattern or morphogenesis. The mutants were ordered into 10 different phenotypic classes. A total of 174 mutants were assigned to 86 complementation groups with two alleles on average. Mutations in 45 complementation groups represent most previously known maternal genes, while 41 complementation groups represent new loci, including several involved in dorsoventral, anterior-posterior, and terminal patterning.     

The Genetic Factors Altered in Homozygous abo Stocks of DROSOPHILA MELANOGASTER

Females homozygous for the maternal-effect mutation abo (2-44.0) produce a large fraction of eggs which arrest during embryogenesis. Increasing doses of defined heterochromatic regions inherited by offspring of abo mothers from their fathers function zygotically to bring about a partial rescue of the abo-induced embryonic lethality. Another property of the abo mutation is that the severity of the maternal effect decreases when an abo stock is maintained in homozygous condition for a number of generations. Here, we show that the factors which change in homozygous abo stocks to result in the decrease in maternally induced embryonic lethality, act zygotically, dominantly and additively. More importantly, we show that the X and second chromosomes, but not the Y and third chromosomes, derived from homozygous abo stocks are, when inherited from males, more effective in promoting zygotic rescue of the abo-induced lethality than are the equivalent chromosomes derived from an abo stock maintained in heterozygous condition. The chromosomal locations of the factors maintained in the homozygous condition. The chromosomal locations of the factors altered in homozygous stock, as well as their behavior, strongly suggest that the same heterochromatic elements that are responsible for rescuing embryos from the abo-induced maternal effect are altered in homozygous abo flies in such a way that the maternal effect itself is less severe.     

Maternal expression increases the rate of <Emphasis Type="Italic">bicoid</Emphasis> evolution by relaxing selective constraint

Population genetic theory predicts that maternal effect genes will evolve differently than genes expressed in both sexes because selection is only half as effective on autosomal genes expressed in one sex but not the other. Here, we use sequences of the tandem gene duplicates, bicoid (bcd) and zerknüllt (zen), to test the prediction that, with similar coefficients of purifying selection, a maternal effect gene evolves more rapidly than a zygotic gene because of this reduction in selective constraint. We find that the maternal effect gene, bcd, is evolving more rapidly than zygotically expressed, zen, providing the first direct confirmation of this prediction of maternal effect theory from molecular evidence. Our results extend current explanations for the accelerated rate of bcd evolution by providing an evolutionary mechanism, relaxed selective constraint, that allows bcd the evolutionary flexibility to escape the typical functional constraints of early developmental genes. We discuss general implications of our findings for the role of maternal effect genes in early developmental patterning.     

Maternal and Zygotic Sex-Specific Gene Interactions in DROSOPHILA MELANOGASTER

Sex-lethal (Sxl) is a vital, X-chromosome gene involved in Drosophila sex determination. The most striking aspect of the phenotype of daughterless (da), an autosomal maternal-effect mutation, may be explained by effects on the functioning of the Sxl gene in the zygote. In this paper, new aspects of interactions between various combinations of Sxl and da alleles are explored in order to understand better the complex da phenotype. The study focuses on the relationship between maternal and zygotic da+ gene functions, and on the relationship between aspects of the da phenotype that are sex-specific and aspects that are not. The SxlM#1 allele, which counteracts the female-specific maternal effect of da, is shown to have no effect on two other aspects of the da phenotype (one maternal, one primarily zygotic) that are not sex-specific. The female-lethal da maternal effect is shown to kill daughters even when the progeny are entirely wild-type with respect to da. Recessive mutant alleles of the two genes can interact synergistically when both are heterozygous with their wild-type alleles, disrupting the development of most of the daughters. Surprisingly, even a deficiency of the da+ locus can produce a dominant, temperature-sensitive, female-lethal maternal effect. A new class of subliminal Sxlf alleles is described. These spontaneous mutations can confuse analysis of both da and Sxl if their presence is not appreciated. Finally, conditions are described that facilitate the study of the Enhancer of daughterless mutation.     

Maternal control of vertebrate development before the midblastula transition: mutants from the zebrafish I

Maternal factors control development prior to the activation of the embryonic genome. In vertebrates, little is known about the molecular mechanisms by which maternal factors regulate embryonic development. To understand the processes controlled by maternal factors and identify key genes involved, we embarked on a maternal-effect mutant screen in the zebrafish. We identified 68 maternal-effect mutants. Here we describe 15 mutations in genes controlling processes prior to the midblastula transition, including egg development, blastodisc formation, embryonic polarity, initiation of cell cleavage, and cell division. These mutants exhibit phenotypes not previously observed in zygotic mutant screens. This collection of maternal-effect mutants provides the basis for a molecular genetic analysis of the maternal control of embryogenesis in vertebrates.     

Environmental and genetic modifiers of squint penetrance during zebrafish embryogenesis

The Nodal-related subgroup of the TGFbeta superfamily of secreted cytokines regulates the specification of the mesodermal and endodermal germ layers during gastrulation. Two Nodal-related proteins - Squint (Sqt) and Cyclops (Cyc) - are expressed during germ-layer specification in zebrafish. Genetic sqt mutant phenotypes have defined a variable requirement for zygotic Sqt, but not for maternal Sqt, in midline mesendoderm development. However a comparison of phenotypes arising from oocytes or zygotes injected with Sqt antisense morpholinos has suggested a novel requirement for maternal Sqt in dorsal specification. In this study we examined maternal-zygotic mutants for each of two sqt alleles and we also compared phenotypes of closely related zygotic and maternal-zygotic sqt mutants. Each of these approaches indicated there is no general requirement for maternal Sqt. To better understand the dispensability of maternal and zygotic Sqt, we sought out developmental contexts that more rigorously demand intact Sqt signalling. We found that sqt penetrance is influenced by genetic modifiers, by environmental temperature, by levels of residual Activin-like activity and by Heat-Shock Protein 90 (HSP90) activity. Therefore, Sqt may confer an evolutionary advantage by protecting early-stage embryos against detrimental interacting alleles and environmental challenges.