By Jhonata Lam
Using common model organisms – Caenorhabditis elegans, Drosophila melanogaster and the like – researchers have uncovered many pathways leading to sexual development. The dissection of procedures responsible for the diversity of different animal systems, however, remains challenging.1
Human physiology is partially determined by the genetic contribution of a pair of chromosomes known as the sex chromosomes. Discovered independently by Stevens and Wilson in the early 20th century,2 the longer and shorter chromosomes of the pair are now termed the X and Y chromosomes respectively. The genotype of a fertilised egg cell possesses a paternal and maternal contribution at the turn of fertilisation, with an XX pair for females and an XY pair for males.
The male-female axis is pivotal to the survival of the human species, and it is partly dependent on the presence of the SRY gene on the Y chromosome.3 SRY expression plays a critical role in the formation of the testis gonad during early embryonic development, consequently letting male-orientated pathways to be implemented.4 The sex determination system in humans is thus known as a Y-centred system. [HA3] [LJ4]
However, this is not a genetic system seen universally. For other organisms, evolution has pioneered disparate genetic solutions to sexual determination.
Referring back to the model insect D. melanogaster, we see another demonstration of the XY system, but with a divergence. Molecularly, this can be traced to the dosage differences imparted by their X chromosome; XX females obtain a double dose of the genes on the X chromosome here, whereas males only receive a single dosage.5 Hence, it differs from the human Y-centred method by being X-centred, with the presence of the X chromosome in this case providing a greater effect in sexual fate.
What is the basis for this dosage dependence? The insect X chromosome harbours the transcription factors runt, sisA and sisB, all of which are doubly expressed in females and lead to sxl splicing factor expression.1This affects the processing of the pre-mRNA of Transformer (Tra) – another splicing factor – via alternative splicing, preventing it from being truncated as in the male pathway.6
Tra has implications in the differential expression of the genes doublesex (dsx) and fruitless (fru), affecting the maturation of sex-specific traits. For example, one group of researchers previously inserted a Gal4 gene into the dsx locus to assist its visualisation and therefore study its function, discerning its importance in the formation of male-specific neuronal circuitry.7 In turn, this was proposed to have had an influence on sexual behaviour.
The honeybee, Apis mellifera, exemplifies how genetics affect complex social organisation. Adopting a system of haplodiploidy, the unfertilised eggs of the bees develop into males8 – a form of asexual reproduction, or arrhenotokous parthenogenesis.9 This means the males remain haploid and so only retain one copy of a set of chromosomes (n). In contrast, females are diploid (2n), so they possess two copies of a set of chromosomes as fertilisation of haploid eggs occur with male haploid sperm.8 Indeed, all female honeybees have a mother and a father, whilst males only have a mother. Subsequently, this forms the foundation of the caste hierarchy determining social roles in a beehive: males are drones and females are either workers or queens. Queens are the only caste that lays eggs and primarily produces offspring. Divergence into either workers or queens is dictated by their food. To produce queens, larvae are fed a special substance called ‘royal jelly’ – without which workers would be produced – that contains proteins like royalactin.10
A. mellifera also utilises splicing in generating sex-specific gene products. Here, it is the heterozygosity of thecsd gene (i.e., possessing 2 different alleles of csd) that prompts the sex-specific splicing of the fem gene.1The Fem protein then goes on to promote normal female development. We may observe an inactive, truncated Fem in haploid males, as heterozygous genotypes are not seen in them.
Furthermore, sex determination mechanisms are not solely confined to genetic factors, as the environment can also hold a significant influence in the growth of some reptiles. They may exhibit sex determination reliant on temperature, for example, a contrast to the prior organisms discussed.
Take the olive ridley sea turtle Lepidochelys olivacea, whose gonadal development is masculinised when temperatures of egg incubation are 26°C and feminised when 33°C.11 Studies elucidating the differential expression of transcription factors during this period were conducted with a technique known as RT-PCR, the role of which is the quantification of mRNA levels. Specifically, researchers assessed the genes Dax1, Dmrt1and Sox9 and ultimately found varying mRNA levels when eggs were incubated with the temperatures above.11 Although results indicated a minor role of Dax1 in temperature-oriented sex determination, they found higher levels of Dmrt1 mRNA in males and higher levels of Sox9 mRNA in females. Altogether, this suggests that temperature-dependent sex determination may be a consequence of inequalities in transcription factor concentrations in a critical period of development.
In summation, organisms have evolved diverse practices to define the male-female axis. Many of these are yet to be revealed in their entirety – environmental sex determination being one example.3 It is therefore a task for future geneticists to elaborate on these mechanisms such that others may fully comprehend their complexity.
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