Variability of evolution: live birth and egg laying simultaneously in lizards of the species Saiphos equalis

Evolution is a delicate matter. The slow and complex process of adapting an organism to environmental conditions, the appearance of some and the extinction of other species, genetic mutations, etc. We often hear the word "evolution", but the process itself, in view of its duration, cannot be observed personally. We can only consider the fossils and compare them with modern organisms, looking for common features and differences that have arisen over time. Or compare different species from the same family living in different regions. Evolution is also not without a sense of humor, because how else to explain the existence of platypuses, kangaroos, drop fish, harpies, unicorns, etc. So, someone here is clearly superfluous.

Time, as we are used to understanding it, is linear and moving forward, like evolution. However, not all organisms like it, so some take a step or a few steps back, returning to the previous stages of their development. This process is called re-evolution. The most curious are creatures that are somewhere outside all of these concepts. These include lizards of the species Saiphos equalis, females of which are capable of both laying eggs and giving birth to cubs. Such maternal uncertainty, which is surprisingly not distributed among individuals, but may be inherent in the same female: if she wanted, she made a laying, she wanted, she gave birth. Today we will get acquainted with an observation in which we first studied this feature in detail. What structural changes are present in the shell, why are scientists not ready to attribute this species of lizards to re-evolution and under what conditions do females prefer egg laying rather than childbirth? Answers await us in a study group report. Go.

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Before you start looking for the answer to the question “what happened before - an egg or a lizard”, it’s worth a little better to find out the central figure of this study. Saiphos equalis is the only species of the genus Saiphos 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 you have very poor eyesight. And all because they have very small paws with three fingers on each. The average length of an individual of this species is 18 cm from nose to tail.

First mention of this endemic *Australia dates from 1825, authored by British zoologist John Edward Gray, who originally called this species “Seps equalis”. Mr. Gray carried it to the genus Saiphos.


John Edward Gray

Endemic * - a representative of flora or fauna that lives / grows exclusively in one limited region. For example, platypuses in Australia, the Baikal seal on Lake Baikal, a tuberous frog in Japan (thanks to people who came to Hawaii, here you have a traveling frog), etc.

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

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


The second name Saiphos equalis is the yellow-bellied three-fingered skink. Photographer's comment: Not dead - just cold.

Viviparous females give birth to fully formed cubs in a transparent membrane. Oviparous females lay full eggs (there is a hard shell), which contain embryos that hatch at the end of formation. The third type with a longer incubation period (15 days) differs from the previous one only in a stronger egg shell.

However, this study would not be so shocking for zoologists if it were not for one nuance - live birth and egg laying together during the same period of pregnancy in one female. And knowing that these two processes need different organs and biochemical processes, it becomes clear why such a phenomenon is so surprising.

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

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

Observation Preparation

In the observations of scientists, 40 Saiphos equalis females collected from the high northern regions and southern lowlands took part. In other words, of the two groups according to the method of birth - live-bearing in the north and ovipositing in the south. Females were kept in the laboratory, taking into account everything necessary: ​​temperature conditions (17–26 ° C); peat moss substrate (55 mm); photoperiod corresponding to the natural habitat; water and food (crickets).

The main tool for comparative analysis of the structure of the shell, membrane and embryos was a scanning electron microscope (SEM). The images obtained with 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 an increase of 1000-9000 times. For data analysis, ImageJ software was used.

Observation Results

As we already know, among all the subjects, the scientists noted one female. Even before the birth of all the other 39 females, she gave birth to 2 eggs, and after one more day. The eggs were covered with a thin transparent membrane. One was used for the initial 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 standard data regarding embryonic development (egg at stage 38-39).


Pictures of embryos: A - embryo (stage 33) inside a thin transparent membrane from a viviparous female; B — hatching of the same embryo (stage 40) after incubation; C - embryo of an “unusual” female taken at stage 33 (egg white was removed).

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

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

Now it was necessary to understand the difference between embryos and shells in females that implemented a particular method of reproduction.


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

Oviparous S. equalis

The egg-laying 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). External fibers are the thickest and contain globular inserts (marked with an arrow for 1s).

It is worth noting that between the outer crust and the membrane is another thin additional membrane. Pay attention to images 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 thin 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).

Viviparous S. equalis

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

S. equalis mixed reproduction method

Shell morphology ( 1f ) in this case is identical at stage 33 (immediately after oviposition) and stage 40 (after hatching). The separation 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 shell layers and its total thickness in such "eggs" is similar in performance to normal eggs in the laying eggs at the developmental stage 36. Moreover, they are structurally similar to membranes in viviparous lizards. In other words, the shell / membrane in lizards with a mixed reproduction method has the properties of a shell in oviparous and a membrane in viviparous at the same time.


Image No. 2: measurement and comparison of the thickness of shells, membranes and intermediate layers.

Chart 2athese are indicators of the thickness of the outer layer and the total thickness at the limiting stage of embryo development. Measurements were carried out 10 times for each sample in three different regions (different points on the shell): oviparous - 36/37 and 39, viviparous - 36 and 39/40, mixed reproduction - 33.

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

For a more detailed study of the study, I strongly recommend that you look into the report of scientists .

Epilogue

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

In the future, the study of this phenomenon will be continued with emphasis on identifying factors that influenced such a re-evolutionary shift. Scientists will also look for the answer to the question - is it re-evolution or is it just a transition period, which will end with a complete transition to live birth, as planned. In addition, such observations give us more information regarding evolutionary processes in general, since we can observe them with our own eyes.

Nature does not cease to amaze, and sometimes shock us with its unpredictability. Each new discovery gives us one answer and another hundred questions to which we, as one of the most curious creatures on the planet, simply must find the answers.



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