3. An evolutionary tree built for eleven species of Pheidole.
At first glance, the animal does not look particularly interesting, however, it is probably one of the most ancient evidences that the precursors of mammals – therapsids – also contained various rudimentary organs in the skeleton (like the wings of a kiwi bird, miniature pelvic bones of a whale, or human tailbone), inherited from even more distant ancestors. We are talking about additional toes on both front paws of the Karenite, which turned the quite typical five-fingered limb of a terrestrial four-legged into a much more elaborate seven-fingered version!
The remains of Karenite were described in 1995 by the outstanding Russian paleontologist Leonid Petrovich Tatarinov. He named this therocephalus in honor of Karen Evgenievich Bogachev, the head of the Moscow cooperative “Stone Flower”, who in 1989-1991 was engaged in geological excavations in the vicinity of the Kirov town of Kotelnich. At that time, the situation was not very good with funding, so some of the found fossils and minerals – the most massive and not interesting for science samples – were sold abroad, and with the money raised, scientists were purchased with everything necessary to continue the work. Fortunately, the karenite discovered in 1991 did not fall into the category of “mass” (not a single complete skeleton was found), much less “uninteresting”, and therefore ended up not in a foreign museum or in private hands, but in the Paleontological Institute of the Russian Academy of Sciences, after which the official description of the new genus of Permian therapsids became known to the entire scientific world.
Reconstruction of the Karenite skull. Pay attention to the pits on the upper jaw – thanks to such sculptural bones of the skull, the karenite received the specific name “decorated”. Drawing from an article by M. F. Ivakhnenko, 2011. Permian and Triassic Therocephals (Eutherapsida) of Eastern Europe
A number of unusual features are observed in the structure of the skeleton of the Karenite, convergently similar to those in reptiles. Most of these features relate to the structure of the wrists and tarsus, but one of the most unusual (and most noticeable) features is associated with the number of toes on the front paws of the animal – their number is two units higher than the generally accepted norm of five fingers. True, you may not notice this right away: two additional fingers, the pre-large (praepollex) and post-mysean (postminimus), are relatively small (even in the pre-large as many as two phalanges!) And during life they were probably hidden under the skin and muscles, like slate bones the horse. It’s just that in a horse such fingers are a real rudiment and a source of uninvited sores like chronic periostitis, while their development in a karenite (if, of course, it was a species trait, and not an anomalous feature of this particular individual) leaves more questions than answers.
In the end, pentadactyly (see Pentadactyly) – the ancestral presence of five fingers on the limbs for all modern vertebrates – more or less settled among tetrapods even in the early Carboniferous period, a hundred million years before the appearance of Karenite, and although among amphibians and reptiles they possess “ there were not so few extra fingers at all times, within our evolutionary line they are quite rare (recall that therocephals, to which the karenite belongs, are a sister group in relation to the direct ancestors of modern mammals). Among modern mammals, they also exist – for example, in rodents from the genus Cape sandworms (Bathyergus) there is a sixth finger, a post-mysenian in the hand, and in striders (Pedetes), the fore-thumb is also equipped with a claw! However, in most members of our class, including humans, such rudiments appear only in the early stages of embryonic development, after which they happily turn into bones of the wrist.
Drawings of the postcranial skeleton of the karenite: A – right hand from above (Prepol. – fore-thumb, Pm. – postmyzinian), B – right hind limb, C – “carapace”. Drawing from L. P. Tatarinov, 2004. The postcranial skeleton of the Late Permian Scaloposaurian Karenites ornamentatus (Reptilia, Theriodontia) from the Kirov Region
Another interesting feature of the karenite was considered to be the presence of a “dorsal carapace” – an unpaired row of wide cutaneous ossifications along the spine. If this were the case, the little therocephalus would become the first among the fossil synapsids to have the outer armor that armadillos and pangolins can boast today. But, as is often the case in paleontology, the original interpretation of unusual bone formations was not confirmed: when re-examining the original fossil, it turned out that the “dorsal shields” are in fact displaced fragments of the supra-sternum – a special unpaired bone, which of all modern mammals is present only in monotremes. while in marsupials and placentals, its functions are performed by a sternum, similar in shape, but different in origin.
For the rest, the karenite was not a particularly remarkable animal, and even against the background of its famous contemporary and compatriot Gorynych (see Gorynych and the bat – new predators of the Permian period from the banks of Vyatka, “Elements”, 06/20/2018) it looked like a dwarf at all: its length the skull was only about 9.5-10 centimeters, and the total body length is estimated at about half a meter. In general, the therocephalus resembled a kind of earless large-headed ferret, possibly even covered with sparse hair – after all, later therapsids definitely had it (see the picture of the day of Coprolites and the great extinction). It is assumed that it was a semi-aquatic or semi-aquatic animal that lived in dense coastal thickets; Perhaps, the way of life of the karenite was somewhat reminiscent of modern minks. It ate relatively small prey (judging by the structure of its teeth – a variety of invertebrates) and, most likely, could equally effectively find food for itself both in water and on land.
Reconstruction of the appearance of the Karenite. Drawing by Dmitry Bogdanov from his book “Lizardmen and other Perm monsters”
Unfortunately, despite the relatively good preservation, so far only fragments of the Karenite skeleton have been found, which is why some of its structural features (for example, the exact size) are still questionable. In many respects, it was a bizarre animal, combining both archaic and rather specialized structural features: well-developed palatine teeth – with three-apical canine teeth, adapted to feed armored prey, and non-fused elements of the cervical vertebrae – with the formation of a special “auditory apparatus” of the lower jaw …
There is nothing particularly strange in this “chimerism” – rather, it is the norm for almost all types of fossil animals, in which, for example, a primitive brain can be combined with a highly specialized dental system – however, it is precisely because of this that it is rather difficult to assess the position of the Karenite on the evolutionary stairs. Just looking at this little therocephalus and his strange fingers on his front paws, who should we consider him – a descendant of a special seven-fingered line of development of synapsids? A genetic freak with a sudden atavism? The answers to these questions, alas, still rest in the depths of the earth.
Image © Smokeybjb from ru.wikipedia.org.
Fig. 1. Castes in Pheidole morrisi ants (this species does not have super-soldiers). A is a winged queen, B is a soldier, C is a worker. D – F – schematic representations of larvae of the three castes in the later stages of development. The larvae that are going to become queens are the largest (D), from the smallest larvae are workers (F). Wing discs (wing buds) are shown, on which areas of expression of the sal gene are indicated in lilac color. Asterisks indicate the absence of wing disks. Below is a diagram of the development of a larva with two forks, at each of which development can follow one of two alternative paths. The choice in both cases is determined by the level of juvenile hormone (JH). Image from the discussed article in Science
Several species of ants from the vast genus Pheidole, in addition to ordinary workers and soldiers, have a caste of “super-soldiers” that protect the colony from the raids of nomadic ants. Scientists from Canada and the United States have shown that if the larvae of those Pheidole species that do not have super-soldiers are treated with juvenile hormone, the larvae turn into a super-soldier. In some of these species, anomalous super-soldier-like specimens rarely occur in nature. Probably, the potential ability for such a “morphosis” (the development of an altered phenotype with an unchanged genome) is inherited by all Pheidole from a common ancestor, although in most species it manifests itself only as a rare anomaly. In species attacked by nomadic ants, the presence of such anomalies was found to be beneficial, and selection reinforced this phenotype by making its appearance in colonies on a regular basis. This evolutionary mechanism, known as “genetic assimilation of morphoses,” explains the independent development of the super-soldier caste in several evolutionary lineages of ants.
The division into castes in social insects is a vivid example of polyphenism. This is the name of the situation when one and the same genotype ensures the development of several discrete phenotypes, and the choice of one of the options depends on external conditions (see: A caterpillar that changes color when heated, “Elements”, 02/09/2006). For example, in ants from the same larva, depending on the conditions (primarily on nutrition), either a winged uterus or a wingless working individual develops.
Representatives of the widespread genus Pheidole, which includes about 1,100 species, besides the usual small workers responsible for collecting food and building work, have argumentative essay outline a second wingless caste – large soldiers, whose tasks include protecting the nest and crunching hard seeds, which form an important part of the diet of these ants. It is possible that the presence of two wingless castes ensured the evolutionary success of the genus, allowing the establishment of an effective division of labor in the colony.
Eight species of Pheidole, living in the deserts of the southwestern United States and northern Mexico, have a third wingless caste – “super-soldiers”, which are even larger in size and have a huge head. The function of the super-soldiers is to protect the colony from the raids of nomadic ants. Super Soldiers guard the entrances to the underground nest, plugging them with their massive heads.
In the colonies of some Pheidole species that do not have this caste, anomalous large individuals with small forewing primordia, similar to oversoldiers, are occasionally found. Biologists from Canada and the United States have suggested that these “freaks” are formed on the basis of the same genetic development program as the real super-soldiers. A larva about to become a soldier differs from a worker’s larva in two ways: firstly, it is larger, and secondly, a pair of wing discs is formed in it (this is the name of the wing buds in larvae). The larva, about to become a uterus, has two pairs of well-developed wing discs; in the larva that “chose” the path of the worker, wing disks are absent (Fig. 1). In addition, the larvae differ in the expression pattern of the important homeotic gene sal, which regulates wing development. In queens, this gene is expressed in two regions of the wing disc: the one that forms the hinge of the wing base (hinge), and the one from which the wing plate (pouch) will develop. In soldiers, this gene is active only in the first of the two areas.
The authors suggested that the super-soldier development program could be formed from the conventional soldier development program by increasing its differences from the worker development program. In other words, in super-soldiers, as compared to soldiers, the larvae should, firstly, grow faster and reach larger sizes, and secondly, they should have more developed wing disks with a pronounced expression of the sal gene. These assumptions were brilliantly confirmed in the course of studying the development of two Pheidole species with a super-soldier caste: P. obtusospinosa and P. rhea (Fig. 2).
Fig. 2. Ordinary soldier (SD) and super-soldier (XSD) P. obtusospinosa. Soldiers develop from smaller larvae (F) with primordia of only front wings (G), in which the sal gene is almost not expressed in the region from which the wing plate develops in winged individuals (H, marked with a black arrow). Oversoldiers develop from larger larvae (J) with two wing discs (K) and with a more pronounced sal (L) expression. Image from the discussed article in Science
The authors built an evolutionary tree for 11 Pheidole species, for which they managed to obtain data on nucleotide sequences (using the sequences of three mitochondrial and two nuclear genes). Of these 11 species, only two (the aforementioned P. obtusospinosa and P. rhea) have a super-soldier caste. Judging by the structure of the tree, the super-soldiers in these two species evolved independently, as a result of parallel evolution (Fig. 3).
Fig. 3. An evolutionary tree built for eleven species of Pheidole. Two species have the super-soldier caste (natural XSD); in three species, it was possible to obtain a super-soldier artificially by treating the larvae with an analog of the juvenile hormone (induction). Shown are the wing discs of the larvae of soldiers (SD) and oversoldiers (XSD); the sal gene expression zones are indicated as in Fig. 1. Drawing from the discussed article in Science
Based on the data obtained, the researchers suggested that the potential for the formation of super-soldiers was inherited by ants of the genus Pheidole from a common ancestor that lived 35-60 million years ago. It was implemented only by those species for which it somehow turned out to be beneficial (for example, because of life in places where nomadic ants are found). In other species, this possibility has been preserved in a latent state. In this case, one should expect that from the larvae of those species that do not have a super-soldier caste, it is possible, by choosing the right conditions, to grow something similar to them.
It is known that the choice of a particular variant of development by an ant larva depends on the level of juvenile hormone (Fig. 1). Therefore, it is logical to assume that with the help of this hormone it is possible to “turn on” the hidden program of development of super-soldiers in species that do not have this caste. Experiments on three Pheidole species without super-soldiers (P. spadonia, P. morrisi, P. hyatti) confirmed this hypothesis. It turned out that if you take a larva of any of these species, about to turn into an ordinary soldier, and smear its abdomen with methoprene (see Methoprene), an analogue of a juvenile hormone, then the larva accelerates its growth, acquires two pairs of wing discs with high expression of the sal gene and eventually becomes a super-soldier (Fig. 4).
Fig. 4. A common soldier P. morrisi (A) and a super-soldier artificially obtained by treating the larva with methoprene (B, C). The white arrows show rudimentary wings, a hallmark of artificial super-soldiers. Image from the discussed article in Science
As already mentioned, in some species that do not have a super-soldier caste (including P. morrisi), such individuals sometimes appear as a rare developmental anomaly (“morphosis”). This allowed the authors to suggest that the independent emergence of the super-soldier caste in different evolutionary lines of ants of the genus Pheidole was due to a mechanism known as “genetic assimilation of morphoses” (see.