Squinting bush brown, Bicyclus anynana
© Judy Gallagher (CC BY-SA)
Once sexual reproduction had become established in multicellular organisms, there was selection pressure to determine the gender of a developing embryo, so the result was either genetically male or genetically female. In humans and other mammals, for example, this is achieved by the XY Chromosomes, which, unlike all the other chromosomes (autosomes) are not paired. A zygote with 2X (homozygous) becomes a female and a zygote with XY (heterozygous) becomes male. Because the zygote gets either one or the other of these chromosomes from each parent these are the only combinations possible, so we never see a YY zygote.
Tell me about sex determination in organisms. Sex determination in organisms refers to the process by which an individual's sex is established. It's crucial for reproduction and can vary greatly across different species. Here are some common mechanisms:In birds, we have the ZW system which works very much like the XY system in mammals, but a female is heterozygous (ZW) while a male is homozygous (ZZ). Again, there can be no WW combination because of the way the chromosomes are inherited from the parents.These are some of the main mechanisms, but there are exceptions and variations within each category. The diversity of sex determination mechanisms across different organisms reflects the complexity of evolutionary processes and the adaptive strategies employed by various species.
- Genetic Sex Determination (GSD):
- This is the most common form, where an individual's sex is determined by their genetic makeup, usually involving sex chromosomes.
- In mammals, including humans, females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
- In birds, the opposite holds true, with females being ZW and males ZZ.
- There are exceptions to this pattern, such as in some insects and reptiles, where the presence or absence of certain chromosomes or combinations of chromosomes determine sex.
- Environmental Sex Determination (ESD):
- In some species, environmental factors, such as temperature during a critical period of development, determine an individual's sex.
- For example, in many reptiles, including some turtles and crocodilians, the temperature at which the eggs are incubated can determine whether they develop into males or females.
- Sequential Hermaphroditism:
- In certain organisms, individuals are born as one sex and later change to the other sex during their lifetime. This can occur either sequentially or periodically.
- For instance, some fish species are born as females and may transition to males later in life, while others may start as males and change into females.
- Social Sex Determination:
- In some species, social interactions within a group can influence an individual's sexual development or expression.
- For example, in certain fish and amphibians, an individual's social status within a group can affect their sex.
- Cytoplasmic Inheritance:
- In certain organisms, factors inherited from the cytoplasm of the egg (rather than from the nucleus) can influence sex determination.
- This is seen in some plants and fungi where certain organelles or cytoplasmic elements carry genes that determine sex.
Approximately, how many different ways are there?
It's challenging to provide an exact number because the ways organisms determine sex can be quite diverse and often overlap between categories. However, based on the major mechanisms mentioned earlier, there are roughly four to five primary ways, including genetic sex determination, environmental sex determination, sequential hermaphroditism, social sex determination, and cytoplasmic inheritance. Within each of these categories, there can be further variations and complexities, making it difficult to quantify an exact number.
In many insects, especially the social insects - wasps, bees and ants - and some beetles, gender is determined by whether the zygote is diploid (male) or haploid (female).
Now, if you believe in the creationist intelligent [sic] design notion, you might think that three separate ways of doing the same thing was more than enough. In fact, of course, any intelligent designer who designed a solution to a problem would stick with that solution, only trying to improve it, but it's a chracteristic of creationism's putative intelligent [sic] designer that it never does the simple thing, and never remembers the solutions it designed earlier. If the same process is required for another species, or entire order, it will be redesigned from scratch, unlike a real intelligent designer who would reuse processes and methods it designed earlier for the same problems today or in the future.
For such a designer, three different methods would be two too many, but there are lots of different sex-determining methods to be found in nature, and a new one has just been added to the list. And this is one that has caused masses of complexity because of the way it kills some developing embryos.
The system is a modification of the ZW system in which ZW produces females and ZZ (normally) produces males. However, in the small African species, the squinting bush brown butterfly, it depends on how alleles of a particular gene on the Z chromosome are inherited. Only one copy of the gene, known as masculinizer, produces a female zygote, even in the absence of a W chromosome, so the female can be either ZW or Z0, but to produce a normal, healthy male needs not just two masculinizer genes, but the genes must be different alleles; two copies of the same allele of the gene will result in an embryo that begins to develop as a female, but then dies because of an imbalance between the expression of genes on the Z chromosome and genes on the autosomes.
Because of this loss of 50% of the males of pairings between parents carrying identical alleles of the masculinizer gene, there has been strong selection pressure to produce lots of different alleles of this gene, so there are now a very large number of these alleles, serving no other purpose but to reduce the probability of an identical pairing - as the work of a putative designer, this is another layer of complexity to overcome a problem of its own creation.
In terms of evolution, it makes perfect sense, of course, and the system probably originated because the species was host to a parasitic Wolbachia bacterium which is passed down the female line and so has an interest in killing males, so its female hosts have less competition for resources - as is commonly found in several diverse insect species.
This system was discovered by a team of geneticists led by Professor Ilik J. Saccheri of Liverpool University, UK, together with researchers from Czech, Dutch and Singaporean universities, who have published their findings open access in the journal Science Advances. They also explain their work in a Liverpool University press release:
New sex-determining mechanism in African butterfly discovered
In a study of a species of African butterfly, researchers have discovered a previously undescribed molecular mechanism of how the sex of an embryo is initially specified.
Across species, a molecular switch is required to initiate development of a newly formed embryo into a female or a male. The genetic identity of this molecular switch, and how it actually works, differs widely across animal groups, which implies that over evolutionary time the sex determination mechanism is under pressure to change.
In insects, the identity of these primary signals, and how they function, has been described in only a handful of species.
In butterflies and moths, it is the females that determine the sex of offspring. The most common sex determination switch is thought to be a gene (Feminizer) on the female-specific W chromosome, which initiates the female pathway by inhibiting a gene (Masculinizer) on the sex (Z) chromosome that initiates the male pathway.
Working on a species of African butterfly, the Squinting Bush Brown, researchers from the University of Liverpool have discovered a radically different molecular switch that does not require a sex-limited gene or chromosome. Instead, it relies on recognition of sequence differences between the two copies of Masculinizer within genetic (ZZ) males, to produce healthy males.
When an embryo has one Z chromosome, and therefore one copy of Masculinizer, it develops into a healthy female (even in the absence of the W chromosome). However, a cost of this mechanism is that ZZ (genetically male) embryos with identical copies of Masculinizer are sent down the female developmental pathway and die as embryos because of imbalances between the expression of genes on the Z chromosome and autosomes (non-sex chromosomes).
Butterflies who mate with an individual that shares an identical copy of Masculinizer produce 50% fewer sons than non-identical pairings. This makes it highly advantageous to be a rare variant (allele) of Masculinizer, resulting in very large numbers of alleles in these butterflies (205 alleles in a sample of 246 females).
Nature has devised many ways of producing males and females. Our discovery of this primary switch highlights the fascinating diversity of sex-determining mechanisms and underlying evolutionary drivers. We suspect that this alternative mechanism evolved as a response to male-killing bacteria that are known to subvert sex-determination machinery in order to promote their own transmission through host females. In the future, we would like to reconstruct where on the evolutionary tree this alternative sex determination mechanism evolved, explore whether structurally similar mechanisms have evolved independently in other butterflies and moths, and figure out how the Masculinizer same/different mechanism actually functions.
Professor Ilik Saccheri, corresponding author
Professor of Ecological Genetics
Department of Evolution, Ecology and Behaviour
University of Liverpool, Liverpool, UK.
AbstractLeaving aside the malevolent incompetence displayed in designing a parasitic bacterium to kill males of a species, then having to design a work-around so the species doesn't become extinct, where is the intelligence in that solution then killing 50% of males if the 'wrong' solution is selected, then needing to design over 200 different solutions to lessen the chances of selecting the fatal one?
Nature has devised many ways of producing males and females. Here, we report on a previously undescribed mechanism for Lepidoptera that functions without a female-specific gene. The number of alleles or allele heterozygosity in a single Z-linked gene (BaMasc) is the primary sex-determining switch in Bicyclus anynana butterflies. Embryos carrying a single BaMasc allele develop into WZ (or Z0) females, those carrying two distinct alleles develop into ZZ males, while (ZZ) homozygotes initiate female development, have mismatched dosage compensation, and die as embryos. Consequently, selection against homozygotes has favored the evolution of spectacular allelic diversity: 205 different coding sequences of BaMasc were detected in a sample of 246 females. The structural similarity of a hypervariable region (HVR) in BaMasc to the HVR in Apis mellifera csd suggests molecular convergence between deeply diverged insect lineages. Our discovery of this primary switch highlights the fascinating diversity of sex-determining mechanisms and underlying evolutionary drivers.
INTRODUCTION
Comparison of sex determination systems across animal lineages has revealed a pattern of high diversity and evolutionary turnover of the primary signal that initiates female or male development (1, 2). In insects, the identity of these primary signals, and how they function, has been described in only a handful of species (3–9). For instance, in the model lepidopteran Bombyx mori, sex is determined by WZ (female) and ZZ (male) sex chromosomes, where the presence of the W chromosome–linked Feminizer (Fem) locus generates Fem Piwi-interacting RNA, essential for female development, and down-regulates a Z-chromosome–linked gene (Masculinizer), essential for male development (5). Here, we report the discovery of a primary sex determination mechanism in the Afrotropical butterfly Bicyclus anynana that differs fundamentally from the mechanism found in B. mori. This is unexpected because both species share the common WZ/ZZ sex chromosome system of Lepidoptera (10).
Our study was initially stimulated by the detection of a recessive lethal genomic interval on the Z chromosome containing B. anynana Masculinizer (BaMasc), which manifested as a deficit of homozygotes (males) in the adult progeny of daughter-father backcrosses (11). To test the role of BaMasc in this lethality, we excluded the genes other than BaMasc within the lethal interval and performed experiments to determine the association and timing of death during development among three categories of BaMasc genotypes, grouped by zygosity (i.e., hemizygous, heterozygous, or homozygous). We investigated the underlying cause of BaMasc homozygote embryonic death via associations with sex-specific splicing of B. anynana doublesex (Badsx), showing that ZZ BaMasc heterozygotes produce the male Badsx isoform, WZ BaMasc hemizygotes produce the female Badsx isoform, but ZZ BaMasc homozygotes also produce the female isoform of Badsx and fail to dosage compensate. We confirmed the causal link with BaMasc by demonstrating, experimentally, that BaMasc knockdown leads to feminization of Badsx. To identify the regions within BaMasc that control this mechanism, we characterized the distinctive structural features and sequence polymorphism of BaMasc and went on to describe the evolutionary consequences of heterozygote advantage for the haplotype diversity of this region in a wild-caught sample from across Africa. Last, through the detection of reproductively competent females lacking a W chromosome, we confirmed that this sex determination system does not require a sex-specific locus.
And all this because this alleged designer kept on designing different ways to determine the gender of sexually-reproducing species.
Yet, even with examples of blundering stupidity on this scale, if it were true, we still get fools in the social media declaring that intelligent [sic] design is the only notion that makes any sense. That belief probably tells us more about the ignorance and difficulty with joined-up thinking than cdesign proponentsists would like us to know.
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