The Right Temperature for Males
Researchers are still learning about these birth strategies’ developmental requirements and constraints. Consider, for instance, the thickness of an eggshell. Before the egg is laid, a thin shell is beneficial because oxygen must enter the egg through the mother’s bloodstream. However, a thicker shell helps shield against predators in the outside world. Therefore, an egg laid too soon could be too thin to survive, and an egg laid too late might be too thick to meet the embryo’s rapidly increasing oxygen needs. Its a finely tuned balance.
In a paper published in Nature in 2009, Organ and his colleagues demonstrated that before a species could evolve live birth, it probably had to evolve the ability to determine the sex of its offspring genetically. The sex of many creatures is circumstantial: Environmental factors, particularly temperature, can determine whether the embryo develops as male or female. Organs team showed a strong statistical association between using genes to determine sex and giving birth to live young. Its something that biologists knew in an observational way, but hadnt ever been rigorously tested, Organ said.
Consider sea turtles. “Even though they have limited mobility on land, they still visit the beach to deposit their eggs,” he remarked. Because the temperature gradient in the water is much smaller than it is on land, they would be less likely to produce a variety of males and females if they laid all of their eggs there.
However, a marine species can completely adapt to its aquatic life and no longer needs to travel on land once it has evolved the ability to determine sex through genes. “Elite physical adaptations to a pelagic lifestyle evolved in each group, such as the fluked tails, dorsal fins, and wing-shaped limbs of ichthyosaurs [a group of prehistoric marine reptiles],” Organ and his coauthors wrote in their paper. .
The three-toed skink embryo is nearly complete when it is ready to be laid in an egg. Due to the late developmental stage at which egg laying is committed, this species can also give birth to live offspring.
At the time of that publication, scientists thought that live birth might have evolved among the reptilian ancestors of ichthyosaurs only after they moved from the land to the sea. But the discovery of a 248-million-year-old fossil changed that. In a paper published in PLOS ONE in 2014, researchers describe the fossil of an ichthyosaur that died while giving birth. Amazingly, the fossil captured the precise moment when the newborn emerged from its mothers pelvis headfirst. That position is telling: Most viviparous marine reptiles are born tail first so that they can continue to draw oxygen from their mother during labor. The headfirst birth position indicates the ichthyosaur inherited live birth from an even more ancient land ancestor. Land reptiles may therefore have been giving birth to live young for at least 250 million years, though the oldest fossil of live birth on dry land doesnt date nearly that far back.
Eggs, Babies or Both
A species may appear to have to choose between giving birth to live offspring or laying eggs, but surprisingly, this isn’t always the case. The Australian three-toed skink (Saiphos equalis), which Whittington and her colleagues study, is a unique lizard in that it can both lay eggs and give birth to live young. There are a few other lizard species that can do both, but in Whittington’s lab, the researchers saw a three-toed skink give birth to a litter that included three eggs and one live young. We were absolutely flabbergasted, Whittington said.
Recently in Molecular Ecology, Whittington and her team describe the differences in gene expression which genes are switched on or off between a lizard mother that lays eggs and one that gives birth to live young. Within a single species, there are thousands of such differences between a female with an egg and one without. Thats because certain genes get switched on when its time for the uterus to house an egg. The same goes for a uterus thats sheltering an embryo. Crucially, the specific genes that get switched on in these cases are very different.
However, in three-toed skinks, many of the genes that become active when a mother produces an egg also become active in mothers who carry embryos. The discovery suggests that this lizard is in a phase of transition between laying eggs and reproducing.
It’s hard to say and might not be clear which way the lizard is evolving. Whittington claimed that evolution is a random process as opposed to a directed one. “Changes in the environment have the potential to revers the direction of selection.” .
The idea that the skink could be moving away from live bearing and back to egg laying is a relatively new development in the field. Twenty years ago we thought it was difficult or impossible for egg laying to re-evolve, Whittington said. But a growing body of research since then has shown that it may be quite common. Recent analyses of genetic relationships between species revealed that certain egg layers are deeply nested within an evolutionary tree of live-bearing neighbors.
Whittington’s motivation for his work is his curiosity about the commonalities among various live-bearing species. “What genetic toolkit allowed for live birth?” she enquired. “Does it use the same genetic instructions when it evolved? Do [different species] have the same problems?” “Are there fundamental rules about viviparity?”
She studies many fascinating creatures in her quest for answers, in addition to the three-toed skink. The only known animal in which males become pregnant is the sea horse. The female deposits her egg into the pouch of her mate to facilitate fertilization and growth. Whittington claims that it is amazing that his research with sea horses has shown that the males activate the same genes that females of many other species do in order to bear live young. Were talking about different sexes. Were talking about completely different tissues. We’re discussing a trait that evolved millions of years ago in two entirely different species,” she said. “It’s similar to having these incredible, millions of year-long naturally reproducing evolution experiments.” The Quanta Newsletter.
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