Yellow crazy ant is a danger to agriculture. More invasive thanks to a new reproductive mechanism

As we learn from the first biology courses in school, multicellular organisms develop starting from a single cell with a precise genetic makeup, and are therefore made up of all cells that are clones of that original cell. In sexually reproducing organisms, the original cell is actually replaced by a fertilized gamete, or by the union of two different cells that each contribute at least one copy of a complete genome; in any case, the resulting organism will always be made up of cells all with the same genetic makeup, derived by division from the fertilized oocyte.

There are, it is true, phenomena of genetic mosaicism in many organisms, including human beings, in which in the adult individual one can find cells that have a partially different set of DNA; in cats, for example, the so-called “tortoiseshell” coat is observed in females because in the different cells, one or the other of the X chromosomes becomes inactivated, if they have a gene that expresses a different version of the color of the coat (orange and black), zones of different colors are generated depending on which of the two remains active.

However, this always concerns a few and selected characters, and not entire genomes; that is, individuals made up of cells with entirely different genetic makeup are not observed.

For the first time since the genetic foundations of sexual reproduction were defined, a notable exception to the above rule has been found: that is, a species has been found in which chimerism is total, presenting individuals made up of cells with a completely different set of DNAand in which it is a systematic phenomenon, with a precise adaptive meaning.

The species is a highly invasive ant, known colloquially as yellow crazy ant due to its frantic behavior when disturbed (Anoplolepis gracilipes).

The discovery, just published on Sciencealso implies a new mechanism of sexual reproductiona phenomenon that will require textbooks to be updated.

To understand what it is, we need to make some general considerations on the genetics of ants.

These animals have two types of genetic makeup: one of type R and one of type W.

Queens with R kit lay R-type eggs. These can be fertilized by R-type or W-type sperm. When an R-egg is fertilized by an R-sperm, a queen develops; when the sperm is type W, however, two different things can happen.

In a first process, analogous to the one we are used to knowing, the nucleus of the sperm W merges with that of the egg R, thus generating a worker.

In a second type of process, the sperm does not fuse with the oocyte, and begins to divide independently of this; in this way completely chimeric individuals are obtained, formed by cells with only R-type DNA and others with only W-type DNA, which will become reproductive males.

In other words, this species is characterized by three types of individuals: individuals whose cells are all RR, corresponding to queens, individuals with all RW cells, corresponding to workers, and R+W chimeric individuals, corresponding to males.

Compare this situation with that of our species, in which we have individuals with all cells of the XX type (women) or of the XY type (men), with the differences concentrated only on a pair of chromosomes, and you will understand the enormous difference and the scope of a mechanism that produces not two, but three types of different genetic makeup as a result of sexual reproduction.

What is the adaptive value of such sexual genetics? Well, the queens of ants (and many other social insects) keep for life the reserves of sperm they accumulate during mating. Thus, a single queen can regenerate all three types of individuals a colony needs – three types, not two as in the case of our species – which also explains the particular invasiveness of these “crazy ants”.

Now, this discovery may seem simply one of those umpteenth new oddities that the natural world holds for us, of the utmost importance for biology textbooks; but here are some data for reflection.

In Australia, the yellow crazy ant landed less than a century ago. Where its numbers reach supercolony levels, it leaves entire forests silent and becomes a major threat to people and pets: does not bite, but sprays formic acid, with which it causes chemical damage (blinding its prey). Yellow crazy ants are also a huge threat to agriculture, because they breed sugar-secreting mealybugs and encourage the growth of sooty molds that can dramatically reduce the productivity of crops such as fruit trees and sugar cane. A study by the University of Melbourne found that, without the current eradication programme, socio-economic costs in one Australian region alone would exceed $500 million over the next 30 years, also jeopardizing tourism values ​​in the region of Queensland’s Wet Tropics World Heritage Rainforests, which makes $2 billion a year.

An invader whose males are constitutively genetic mosaics may seem irrelevant just because it’s an ant, but when its species is on the world’s 100 most invasive list, any additional information about it has a value beyond that of basic research. , being able to provide new avenues for control – for example through interference with its complex method of reproduction.