Female iguanas often lay eggs without encountering a male, a normal biological process for many reptiles. These eggs are typically unfertilized. However, in extremely rare instances, an iguana that has not mated can produce fertile eggs that hatch viable young. This phenomenon is a form of asexual reproduction.
Understanding the Difference: Unfertilized Eggs
The majority of eggs laid by an isolated female iguana are not capable of developing into offspring. Female iguanas undergo an annual reproductive cycle, resulting in the formation and deposition of eggs regardless of whether mating has occurred. A mature female green iguana will lay a clutch once a year, sometimes containing between 20 and 70 eggs.
These infertile eggs are often referred to as “duds.” The female requires significant energy and calcium to produce the shells and yolks for these eggs. Laying these eggs is simply a hormonal function, and they will not hatch. Owners of pet iguanas who find eggs without a male should assume they are infertile and dispose of them.
Parthenogenesis: The Biological Mechanism
The rare occurrence of a viable egg laid without mating is known as parthenogenesis, a form of asexual reproduction. In this process, an egg develops into an embryo without the genetic contribution of sperm. This adaptation is observed in certain lizards, snakes, sharks, and some birds, but it is not a routine event in iguanas.
In iguanas, this process is usually a type of parthenogenesis called automixis, specifically involving a mechanism known as terminal fusion. During the formation of the egg cell in the female’s ovary, the cell divides, producing the egg and several small, non-viable cells called polar bodies. In a parthenogenetic event, one of these polar bodies effectively acts as a substitute for sperm.
The polar body fuses with the egg cell, combining their genetic material to restore the full set of chromosomes. This fusion allows the egg to begin dividing and developing into an embryo. This event has been documented in species like the casque-headed iguana, confirming the fertility of eggs laid by isolated females.
Offspring Viability and Genetic Results
While parthenogenesis can result in a viable embryo, the hatch rates are often lower compared to eggs produced sexually. The successful hatching of a parthenogenetic clutch is rare in iguanas, sometimes yielding a small number of hatchlings. For example, one documented case resulted in eight healthy babies from an isolated female.
The resulting offspring are not identical clones of the mother, but they are highly genetically similar. This is because the fusion of the polar body with the egg cell creates high homozygosity, meaning the offspring have two copies of the same or very similar genetic instructions. Since the offspring inherit genetic material only from the mother, they lack the genetic variation introduced by a male.
A significant genetic outcome of this reproductive strategy in iguanas is that the offspring are almost exclusively female. The specific sex determination system in these reptiles causes the fusion event to primarily result in female embryos. These new females are then capable of reproducing sexually if they encounter a male, or they can potentially reproduce asexually again, continuing the lineage without a male partner.