ARTICLES
The first known case of Eggs plus Live Birth from One Pregnancy in a tiny Lizard (continued)
Mothers know best
A live-bearing female S. equalis in our laboratory colony laid
three eggs, one of which hatched to produce a healthy baby.
The placenta is a highly complex organ. One of its jobs is to
transfer nutrition to the developing baby.
However, our latest research shows this lizard is intriguing in
another completely unexpected way.
Despite both these complex steps, reptiles, particularly snakes
and lizards, appear to be unusually predisposed to making this
leap to live birth. This capacity has evolved in at least 115 groups
of reptiles independently.
We observed a live-bearing female that laid three eggs, and then
gave birth to a living baby from the same litter weeks later. We
incubated two of the eggs, one of which hatched to produce a
healthy baby.
Having it both ways
This finding is remarkable for two reasons. Firstly, as far as we
are aware, this is the first example of both egg-laying and live
birth within a single litter for any vertebrate.
It’s easy to see why reptiles, as a group, are fascinating models
for studying how live birth evolves from egg-laying. Of particular
interest are two Australian skinks that have both live-bearing and
egg-laying individuals (known as being bimodally reproductive),
making these lizards incredibly valuable to evolutionary biologists
as they offer a snapshot into evolutionary processes in action.
Secondly, in some cases, individuals may be capable of
“switching” between reproductive modes, in other words, as
laying eggs and giving birth each come with their own specific
advantages and disadvantages, individuals may be able to
“choose” which option best suits the current situation!
The three-toed skink, Saiphos equalis, is one such species.
Reproduction in Saiphos equalis varies geographically:
populations around Sydney lay eggs, while those further north
give birth to live young.
A closer look at eggshells
To better understand this reproductive phenomenon, we then
investigated the structure of the egg coverings of these unusual
embryos in minute detail (using an advanced technology called
scanning electron microscopy).
Whether individuals are live-bearing or egg-laying does seem
to be genetically determined: when researchers swap their
environmental conditions (by transferring them from one site to
another), the females retain their original reproductive strategy.
We found that in this litter, the egg-coverings were thinner
than those of normal egg-laying skinks and had structural
characteristics that overlapped with those of both egg-layers
and live-bearers (which have thinner coverings that are greatly
reduced).
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 3