SEX DETERMINATION: TYPES OF SYSTEMS AND CHARACTERISTICS - BIOLOGY - 2025

The determination of sex is controlled by a number of mechanisms varied between taxa that establish sexual characteristics of the individual. These systems can be intrinsic to the individual - that is, genetic - or be controlled by environmental factors that surround the individual during the early stages of life.

In intrinsic determination, biologists have classified these systems into three main categories: individual genes, haplodiploid system, or special or sex chromosomes. This last case is that of us, mammals, birds and some insects.

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In the same way, environmental conditions also influence sex determination. This phenomenon has been studied in some reptiles and amphibians, which are particularly influenced by temperature. This system of determination is known as cryptic.

Types of sex determination systems

Sex, understood as the mixing of genomes via meiosis and gamete fusion, is a virtually universal event in the life of eukaryotes.

One of the most important consequences of sexual reproduction is the coupling of different alleles, carried by different individuals, in a beneficial genetic variation.

In most eukaryotic organisms, sex determination is an event that occurs at the time of fertilization. This phenomenon can occur by three different systems: individual genes, haplodiploid system, or special chromosomes.

Likewise, we have the determination of sexual characteristics mediated by environmental factors, such as temperature. This occurs in frogs, turtles, and alligators, where the incubation temperature seems to determine sex.

We will describe each system below, using examples taken from the animal and plant kingdoms:

Individual genes

In organisms where sex is determined by individual genes, there are no sex chromosomes. In these cases, sex depends on a series of alleles located on specific chromosomes.

In other words, sex is determined by a gene (or by several of these) and not by the presence of a complete chromosome.

Different vertebrates, such as fish, amphibians and some reptiles have this system. It has also been reported in plants.

The alleles that participate in this phenomenon have the widely known dominance system that exists for autosomal characters. In plants, the alleles that determine masculinity, hermaphroditism and the feminine character of the individual have been specified.

Haplodiploid system

The haplodiploid system is common in bees

Haplodiploid systems determine sex depending on the haploid or diploid condition of the individual. We humans are diploid - both males and females. However, this condition cannot be extrapolated to all animal groups.

The haplodiploid system is quite common in Hymenoptera (bees, ants, and the like), Homoptera (mealybugs and chicks), and Coleoptera (beetles).

The classic example is that of bees and the determination of sex in colonies. The social structure of bees is extremely complex, as are their eusocial behaviors, having their bases in the genetic system that decides their sex.

Bees lack sex chromosomes. Females are diploid (2n) and males are haploid (n), called drones. For this reason, the development of the females comes from the fertilization of the eggs, while the unfertilized ovules develop into males. That is, the latter have no father.

In females, the division between the workers and the queen is not genetically determined. This hierarchy is determined by the individual's diet in the early stages of her life.

Special chromosomes

The case of special chromosomes or sex chromosomes is the one with which we are most related. It is present in all mammals, all birds and many insects, being a common form in organisms with different sexual phenotypes.

In plants, although it is very rare, it has been possible to specify some dioccal species that have sex chromosomes.

This system has different variants. Among the most common and simple we find the systems: XX-X0 and XX-XY, where the heterogametic sex is the male, and the ZZ-ZW, where the heterogametic sex is the female.

The first system, XX and X0, is common in insects of the order Orthoptera and Hemiptera. In these cases, the male has only one sex chromosome.

The XX and XY system is present in mammals, in many insects of the Diptera order and in a very restricted number of plants, such as Cannabis sativa. In this system, sex is determined by the male gamete. If the latter has the X chromosome, the offspring corresponds to a female, while the Y gamete will give rise to a male.

The last system, ZZ and ZW, is present in all birds and in some insects of the order Lepidoptera

Cryptic determination

In certain taxa, the different environmental stimuli, in the early stages of the life of individuals, have a crucial role in determining sex. In these cases, the determination from the genetic point of view has not been completely elucidated, and the sex seems to depend entirely on the environment.

In sea turtles, for example, a variation of an extra 1 ° C turns an entire population of males into a population made up exclusively of females.

In alligators, it has been found that an incubation below 32 ° C produces a population of females and temperatures above 34 ° C translate into a population of males. In the range 32 to 34, the proportions between the sexes are variable.

In addition to temperature, the influence of other environmental variables has been demonstrated. In one species of annelid, Bonellia viridis, sex is determined in its larval stage. The larvae that swim freely in the water, develop as males.

In contrast, larvae that develop close to mature females are transformed into males by certain hormones that they secrete.

Infection by microorganisms

Finally, we will discuss the special case of how the presence of a bacterium is able to define the sex of a population. This is the case of the famous bacterium belonging to the genus Wolbachia.

Wolbachia is an intracellular symbiont, capable of infecting a wide range of arthropod species and also some nematodes. This bacterium is transmitted vertically, from females to their future offspring, by eggs - although horizontal transfer has also been documented.

Regarding the determination of sex in the organisms it inhabits, Wolbachia has highly relevant effects.

It is capable of killing males in the population, where infected males die during the early stages of their life; feminizes the population, where the developing males become females; and finally, it is capable of producing parthenogenetic populations.

All these phenotypes mentioned, which involve the distortion of the sex ratio with a marked bias towards females, occur to favor the transmission of the bacteria to the next generation.

Thanks to its wide host range, Wolbachia has played a crucial role in the evolution of arthropod sex determination systems and reproductive strategies.

Proportion of the sexes

Pack of wolves.

A fundamental property of sex determination systems corresponds to understanding the proportion of the sexes or sex ratio. Several theories and hypotheses have been proposed:

Fisher hypothesis

Ronald Fisher, an acclaimed British biologist and statistician, proposed in 1930 a theory to explain why populations maintain a 50:50 ratio of males to females. Reasonably, it also explained why the mechanisms that skew this equal ratio are selected against.

Consecutively, it was possible to demonstrate that a fair or balanced sex ratio constitutes a stable strategy, from the evolutionary point of view.

It is true that Fisher's results do not apply in certain circumstances, but his hypothesis seems to be general enough that the mechanisms for determining sex should be selected according to his principles.

Trivers and Willard hypothesis

Later, in 1973, these authors noted that the sex ratio depended on many other factors - mainly the physiological state of the female - that were not taken into account in Fisher's explanation.

The argument was based on the following premises: when a female is physiologically "healthy", she should produce males, because these young will have a greater chance of surviving and reproducing.

In the same way, when the female is not in optimal physiological conditions, the best strategy is the production of other females.

In nature, weak females often reproduce, despite their physiological "inferiority" status. In contrast to a weak male, where the chances of reproduction are exceptionally lower.

This proposal has been tested in various biological systems, such as rats, deer, seals, and even in human populations.

Evolutionary perspective and future questions

In the light of evolution, the diversity of the mechanisms that determine sex raises certain questions, including: why do we see this variation? How does this variation arise? And finally, why do these changes occur?

Furthermore, it also arises from the question whether certain mechanisms give the individual a certain advantage over others. That is, if any particular mechanism has been selectively favored.

References

1. Asgharian, H., Chang, PL, Mazzoglio, PJ, & Negri, I. (2014). Wolbachia is not all about sex: male-feminizing Wolbachia alters the leafhopper Zyginidia pullula transcriptome in a mainly sex-independent manner. Frontiers in microbiology, 5, 430.
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