A small number of lizard species have evolved a reproductive strategy where populations are composed entirely of females. These species reproduce without any genetic contribution from a male. This ability allows a single female to establish a new population, doubling the rate of population growth compared to species that require two sexes. Understanding these lizards requires examining the cellular mechanisms and tracing the evolutionary events that led to their origins.
The Biological Mechanism of Parthenogenesis
The process allowing these lizards to reproduce without a male is called parthenogenesis, a form of asexual reproduction where development occurs from an unfertilized egg. Normally, an egg contains only half the mother’s chromosomes, requiring fertilization to restore the full diploid set of genetic material. Parthenogenetic lizards modify this process to create a viable embryo on their own.
The primary mechanism used by obligate parthenogenetic lizards involves premeiotic genome doubling. Before the egg-forming cell undergoes meiosis, it doubles its entire set of chromosomes, resulting in a tetraploid cell containing four copies of the genome. This cell then undergoes two division cycles. Because the chromosome number was initially doubled, the final egg retains the full, diploid complement of chromosomes.
Another cellular strategy is automixis. This process occurs after the standard meiotic division, which results in a haploid egg and a small, haploid polar body. The egg cell then fuses with the polar body, restoring the diploid chromosome number necessary for development. The resulting offspring are essentially clones of the mother, though genetic variation can still occur due to the cellular fusion.
Notable Examples of All-Female Lizard Species
The primary examples of all-female lizards belong to the New World Whiptail Lizards, within the genus Aspidoscelis. Found across the southwestern United States and Mexico, this group includes approximately 15 distinct species that reproduce exclusively through parthenogenesis. The New Mexico Whiptail (Aspidoscelis neomexicanus), the official state reptile of New Mexico, is one of the most famous examples, characterized by seven light-colored stripes.
Eurasian Rock Lizards of the genus Darevskia inhabit the mountainous regions of the Caucasus and eastern Turkey. Several species within this genus, such as Darevskia armeniaca and Darevskia unisexualis, arose through hybridization events. Scientists estimate there are around 50 lizard species that reproduce solely through this asexual method, making them the most common group of vertebrates to exhibit obligate parthenogenesis.
Even without males, these female whiptail lizards engage in behaviors that mimic courtship and copulation, known as pseudo-copulation. One female takes on the male role and mounts another, a behavior driven by hormonal cycles, specifically progesterone. Studies suggest this pseudo-copulation stimulates the hormonal changes necessary to promote ovulation.
Evolutionary Origins of All-Female Lizards
Most all-female lizard species originate from a hybridization event between two sexually reproducing parent species. For example, the New Mexico Whiptail is the product of a cross between the Western Whiptail (Aspidoscelis tigris) and the Little Striped Whiptail (Aspidoscelis inornatus). This initial cross resulted in a hybrid female whose offspring had the ability to reproduce asexually.
The hybridization process is theorized to have led to the breakdown of normal sexual reproduction, allowing the parthenogenetic mechanism to take over. This origin story provides the newly formed asexual species with a rich initial genetic diversity, as it combines the genomes of two separate parent species. This genetic mixing is important because asexual reproduction limits the introduction of new genetic material, which can lead to a long-term buildup of harmful mutations.
The adaptive advantage of this strategy is the ability to colonize new or unstable habitats rapidly. Since a single female can start a new lineage, these all-female species are well-suited for expanding their range into environments where finding a mate is difficult. However, the lack of genetic shuffling over time means these lineages are considered evolutionarily young and may be less able to adapt to significant environmental changes.