The inconstancy of evolution: live birth and egg-laying simultaneously in lizards of the species Saiphos equalis

Evolution is a delicate matter. Slow and complex process of adaptation of an organism to environmental conditions, the emergence of some and the extinction of other species, genetic mutations, etc. We often hear the word "evolution", but due to its duration, we cannot personally observe the process itself. We can only consider the fossils and compare them with modern organisms, looking for common features and differences that have arisen over time. Or to compare different species from the same family living in different regions. Evolution is also not devoid of a sense of humor, for how else can one explain the existence of platypuses, kangaroos, fish droplets, harpies, unicorns, etc. So, someone here is clearly superfluous.

Time, as we used to understand it, is linear and moves forward, like evolution. However, not all organisms like this; therefore, some take a step or several steps back, returning to the previous stages of their development. This process is called re-evolution. The most curious are the creatures that are somewhere outside of all these concepts. This includes the lizards of the species Saiphos equalis, the females of which are capable of both laying eggs and giving birth to young. Such maternal vagueness, surprisingly, is not distributed among individuals, but may be inherent in the same female: she wanted - she laid, she wanted - she gave birth. Today we will get acquainted with the observation, in which this feature was first studied in detail. What structural changes are present in the shell, why are scientists not ready to attribute this type of lizard to re-evolution, and under what conditions do females prefer to lay eggs, and not to give birth? The answers are waiting for us in the report of the research group. Go.
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main character

Before you start looking for the answer to the question “what was before - an egg or a lizard,” you should know a little better the central figure of this study. Saiphos equalis is the only species of the Saiphos genus of the skink family. The family is the richest in diversity, for it has about 130 genera and more than 1,500 species.


Saiphos equalis

Saiphos equalis lizards can be easily mistaken for small snakes, especially if your vision is not very good. And all because they have very small legs with three fingers on each. The average length of an individual of this species is 18 cm from nose to tail.

The first mention of this endemic * Australia dates back to 1825 under the authorship of the British zoologist John Edward Gray, who originally called this species "Seps equalis". Mr. Gray attributed it to the Saiphos genus.


John Edward Gray

Endemic * - representative of the flora or fauna, living / growing exclusively in one limited region. For example, the platypus in Australia, the Baikal seal on Baikal, the lumpy frog in Japan (thanks to the people came to Hawaii, here you have a traveling frog), etc.

The most significant distinguishing feature of this creature is its variation in reproduction. Females of this species can produce offspring in three ways: laying eggs (15 days of incubation period), live birth and intermediate type with egg-laying (5 days of incubation period).

The incubation period of other lizards is much longer, which may indicate the transition of Saiphos equalis from egg-laying to live birth. Moreover, this process is strongly associated with environmental conditions. For example, females from higher mountain regions (from 1000 m above sea level) are viviparous, while females from lowlands lay eggs with a short incubation period.


The second name Saiphos equalis is a yellow-bellied three-toed skink. Photographer's comment: Not dead - just cold (not dead - just cold).

Viviparous females produce fully formed pups in a transparent membrane. Oviparous females lay high-grade eggs (there is a hard shell), which contain embryos hatching upon completion of formation. The third type with a longer incubation period (15 days) differs from the previous one only in more durable eggshell.

However, this study would not have been so shocking for zoologists, if not for one nuance - live birth and egg-laying together in one gestation period in one female. And knowing that these two processes require different organs and biochemical processes, it becomes clear why this phenomenon is so surprising.

A few details about the egg-laying from the researchers themselves. The females of the egg-laying lizard species lay eggs at the embryonic stage 30, the young hatch at stage 40. But the females of the species Saiphos equalis lay at 38-39, the incubation period averages 5.5 days. The difference is quite palpable. Live-born cubs covered with a thin membrane are released from it within 1.5 days after birth.

In their work, scientists describe the observations of the female Saiphos equalis, which in controlled laboratory conditions at stage 33 laid 3 eggs, followed by the birth of a fully formed baby. Scientists compared the morphology of the eggshell of this female and the females that produce offspring by only one method (live birth or egg-laying).

Preparing for the observations

In the observations of scientists took part 40 Saiphos equalis females, collected from the high-mountainous northern regions and southern lowlands. In other words, of the two groups according to the method of birth - viviparous in the north and egg-laying in the south. Females were kept in the laboratory with all the necessary: ​​temperature conditions (17–26 ° C); wet peat moss substrate (55 mm); photoperiod appropriate to the natural habitat; water and food (crickets).

The main tool for a comparative analysis of the structure of the shell, membrane and embryos was a scanning electron microscope (SEM). The images obtained by this microscope made it possible to find out the thickness of the shell (shell or membrane). Each sample was tested 10 times in three different areas using the magnification of 1000-9000 times. Data analysis software was used ImageJ.

Observation results

As we already know, among all the subjects, scientists noted one female. Even before the birth of all the other 39 females, she produced 2 eggs, and a day later one more. The eggs were covered with a thin transparent membrane. One was used for the primary analysis, and the other two were incubated.

It was possible to find out that the embryo in the egg is at stage 33, and this is a much earlier stage compared to the standard data regarding embryonic development (the egg at stage 38-39).


Embryo images: A - an embryo (stage 33) inside a thin transparent membrane from a live-giving female; B - hatching of the same embryo (stage 40) after incubation; C - embryo "unusual" females, collected at stage 33 (egg white was removed).

Unfortunately, one of the incubated eggs died, but the second one developed absolutely normally, and a fully formed and healthy baby was born. 37 days passed from the moment of laying to hatching. The same female, 41-48 days after laying, gave birth to one calf.

To summarize: 39 females gave life to their offspring by one of the methods, 1 female decided to use both and laid 3 eggs, and by one live birth gave birth to one young.

Now it was necessary to understand the difference between embryos and shells in females who implemented one or another method of reproduction.


Image No. 1: Images of SEM shells and membranes.

Oviparous S. equalis

The egg shell of S. equalis consisted of an outer coarse layer, followed by a membrane layer of interwoven fibers embedded in a dense crystalline matrix (1a and 1b). The outer fibers are the thickest and contain globular inserts (marked with an arrow for 1s).

It should be noted that between the outer crust and the membrane is another thin additional membrane. Notice pictures 1a and 1b in which these transition layers are marked with a black arrow.

The inner membrane covers another inner separation layer consisting of a dense matrix of fine fibers. The morphology of the shell between stages 36/37 and 39 is extremely similar, including the ratio of the layers. However, over time, the shell thickness decreases by 45.5% from 32.9 + 1.3 µm (stage 36/37) to 17.9 + 2.5 µm (stage 39).

Vivipars S. equalis

In viviparous S. equalis, the shell (1d, 1e) is structurally very similar to the egg-laying shells. The difference lies primarily in the separation layer between the outer hard shell and the inner membrane. In viviparous, this layer is rather difficult to identify, especially at the stage 39/40 (black arrow on 1e). From snapshot 1d, it can be seen that at stage 36, a membrane of more dense fibers begins to rise to the outer layer, and a layer of thinner fibers is embedded in a dense matrix. The sheath fibers are more uniform in diameter at stage 39/40 (1e). At stage 36, the shell thickness is 11.67 + 1.9 microns, and already at the 39/40 stage - 8.35 + 2.1 microns, that is, the shell becomes thinner by 28.5%. The main reduction in thickness is observed in the membrane layer.

S. equalis with mixed reproduction method

The morphology of the shell ( 1f ) in this case is identical at stage 33 (immediately after egg-laying) and stage 40 (after hatching). Separating inner layer could not be identified. The fibers are fairly uniform and dense, and the thickness is 9.27 + 0.25 microns. The proportionality of the layers of the shell and its total thickness in such “eggs” is similar in performance to normal eggs in egg-laying at the development stage 36. At the same time, they are structurally similar to the membranes in live-lizards. In other words, the shell / membrane in lizards with a mixed reproduction method has the properties of a shell in egg-laying and membranes in viviparous at the same time.


Image No. 2: Measuring and comparing the thickness of the shells, membranes and intermediate layers.

Chart 2a is an indication of the thickness of the outer layer and the total thickness at the limiting stage of embryo development. The measurements were carried out 10 times for each sample in its three different regions (different points on the shell): egg-laying - 36/37 and 39, viviparous - 36 and 39/40, mixed reproduction - 33.

Graph 2b is the ratio of the thickness of the outer coarse layer and the membrane. And graph 2c is the thickness of the outer coarse layer at different stages.

For more detailed acquaintance with the study I strongly recommend to look into the report of scientists .

Epilogue

Observation of egg-laying and live birth in the same female was first described in this very work. According to scientists, the ability to lay eggs in species that are on the evolutionary path to a live birth may mean that there is an opportunity to reverse this process. In other words, S. equalis lizards, which began the transition from egg-laying to live-birth, may, due to certain factors, return to the previous reproduction system. So far, researchers cannot say for sure whether S. equalis will retain a mixed reproduction method, but there is such a possibility.

In the future, the study of this phenomenon will be continued with emphasis on identifying the factors that influenced such a re-evolutionary shift. Also, scientists will look for the answer to the question - whether it is a re-evolution or just a transitional period, which will end in a full transition to a live birth, as was planned. In addition, such observations provide us with more information about the evolutionary processes in general, since we can observe them personally.

Nature never ceases to amaze and sometimes shock us with its unpredictability. Each new discovery gives us one answer and a hundred more questions to which we, as one of the most inquisitive creatures on the planet, simply have to find the answers.



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