In some animals in the wild, males cannot be found at all, and they exist by asexual reproduction. Researchers have discovered that the mites, for example, control the number of males in the population through parasitic bacteria
Dror Bar-Nir, Galileo
Most multicellular and eukaryotic creatures (having a nucleus wrapped in a membrane), including ourselves, are diploid - carrying two sets of hereditary information, each of which came from a different parent. The process in which an individual receives two sets of hereditary information from two parents is called sexual (marital) reproduction. In this process, two individuals of the same biological species (species), usually from two pairs - a male and a female - produce offspring. Haploidy (a single genetic system, meaning one copy of each chromosome rather than two) and asexual reproduction, which is essentially a duplication of the parent (with the exception of a negligible rate of random mutations), were "left" by the multicellular eukaryotic organisms to bacteria and most unicellular organisms.
The "price" for sexual reproduction is very high: instead of the reproduction of the reproducing cell into two identical offspring cells, in a process known as "mitosis", four cells, called gametes or gametes, are created in a process known as "meiosis". The process of meiosis includes a phase called reduction division, in which the cell goes from a diploid state - in which the cell has two sets of chromosomes - to the haploid state, in which the cell has only one set of chromosomes, that is, one copy and not two of each chromosome.
Only a very small percentage of the gametes, or gametes, formed in this way will meet gametes of the opposite sex, and only these few gametes will produce young individuals that will be part of the next generation. Besides the waste of resources involved in the creation of gametes, the individuals must invest a lot of energy in locating a potential mate, courting and sometimes even fighting with competitors for that mate.
As we know, it is accepted that the main advantage of sexual reproduction is in the creation of genetic diversity among the various individuals of the population (in a population where reproduction is asexual there is almost no diversity). If the conditions change radically, the diversity allows at least some of the individuals - those endowed with a suitable combination of genes - to live in the new environment. This advantage of sexual reproduction justifies, according to some researchers, the high "cost" and inefficiency of sexual reproduction.
Intermediate state
An intermediate state, called haplodiploidy, exists in several groups of social insects - ants and bees - where the males are haploid, while the queens and workers are diploid females. Even in these species, it is still about sexual reproduction.
In the hereditary information of every creature that reproduces by sexual reproduction is the information that enables the development of both couples. Until the last few years, two types of mechanisms were known that determine whether a certain individual will develop into a male or a female: the first is environmental and the second is genetic. An example of the first mechanism is the effect of the temperature at which certain crocodile eggs are "incubated" on the mating of the hatched offspring. Another example is certain species of fish, where all individuals are born as females, and in the absence of a male, the dominant female changes her sex. In contrast, in most mammals and most other eukaryotic creatures, the mechanisms that determine mating are genetic. In humans, for example, male sexuality is determined by the presence of a Y chromosome (or a certain gene on it). In other species the pairing is determined by the "quantity" of a gene (or a certain chromosome): a single copy of the gene will determine one pairing, while the existence of two copies will determine the opposite pairing.
Well, it turns out that there are other mating mechanisms as well. Most mites - tiny arthropods from the arachnid series - are haplodiploid (the males are haploid and the females are diploid), and reproduce by sexual reproduction, similar to ants and bees. But the acari of the genus Brevipalpus deviate from this rule: they are haploid, and they breed in virgin reproduction (parthenogenesis) - the laying of unfertilized eggs, from which only the haploid females emerge.
The flat mite (B. phoenicis), which belongs to this genus, has two chromosomes (different, that is to say - non-paired). It lives on at least 65 different plants in tropical and subtropical regions, and causes heavy damage to economically important crops such as citrus, papaya, tea and coffee.
Rare males
Does the mite population include only females? - It turns out that the answer is not both absolute, because from time to time males also hatch from the eggs (they are also haploid), but their rate usually does not exceed five percent.
What is the origin of this relationship? Do the males even participate in the reproductive process of these acarias? Do these bacteria have sexual reproduction, and are their products diploid? Why are there males in the population at all, in a relatively small percentage? In each female, and also in the eggs she lays, several dozen symbiotic bacteria also live. The symbiont bacteria, which are probably obligate (unable to live except inside the host), have not yet been defined.
To test whether the presence of the bacteria has an effect on the biology of the mites, Andrew Weeks (Weeks) and his team, from the University of Amsterdam, treated a group of mites with the antibiotic tetracycline for three days. Another group of mice, which served as a control, was not treated with tetracycline. During the ten days after the antibiotic treatment, all the eggs laid by the mites were collected and the sex of the offspring that hatched from them was examined. The results were impressive: while in the control group the percentage of males was about 6%, about 50% of the offspring of the mites treated with antibiotics were males - a 10-fold increase.
Wicks' conclusion: the bacteria prevent, by an unknown mechanism, the male development of the genetic males, which develop as females.
Sexual reproduction that improves the chances of survival
When males are created, sexual reproduction occurs in a population, which can probably contribute to the diversity of the population (of course only on the condition that the males move to another population, which is genetically different from the population of origin). The formation of males and their genetic contribution to other populations prevents the populations of these acarias from becoming a genetically uniform tribe.
It seems that we have a situation of symbiosis, where both the mites and the bacteria benefit from sharing. The profit of the bacteria is due to the fact that they are passed on to the next generation (through the eggs) in most of the offspring. The mite that carries the bacteria also benefits from this, due to the transition to virgin reproduction and the savings in the "price" of sexual reproduction. This gives it a survival advantage over its competitors that reproduce sexually, as long as the environmental conditions are stable.
The flatfish is not alone. It turns out that bacteria of the genus Wolbachia, which exist as intracellular parasites in arthropods and other small creatures, cause, among other things, the death of male embryos still in the eggs and/or the transformation of genetic males into females (feminization). The Wolbachia bacteria that are housed in isopod crustacean species (Isopoda, which include the spp.
As a result of the infection, the genetic males become phenotypic females, who will also pass the bacteria on to the next generation. Hence, a new and interesting mechanism is added to the mating determination mechanisms we described at the beginning of the column - bacterial infection.
* Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University.
