Reproduction and Mating Behaviors in Lizards and Birds

Asexual Reproduction and Mating Behaviors in Lizards

Some lizard species reproduce asexually, a process where offspring are produced without fertilization by a male. Interestingly, certain asexual species, such as the New Mexico whiptail lizard, still engage in mating behaviors. These behaviors are not for reproduction but are believed to stimulate ovulation and increase fertility. During these interactions, one lizard assumes a role similar to that of a male and mounts the other, which is hormonally influenced. The lizard that has just laid eggs will have higher levels of progesterone, which is associated with male-like behavior, while the one about to lay eggs has higher estrogen levels, prompting the female role. These pseudo-sexual behaviors may enhance reproductive success, as lizards that engage in them tend to have higher egg production than those that do not.

Geographic Parthenogenesis in Lizard Populations

Geographic parthenogenesis refers to the distribution pattern where asexual species are found in environments that are less hospitable to their sexually reproducing relatives. This is exemplified by certain parthenogenetic lizards, such as the Caucasian rock lizards of the genus Darevskia. These lizards, which reproduce through parthenogenesis, are often found in high-altitude environments where their sexually reproducing ancestors are less successful. For instance, the parthenogenetic lizard Darevskia dahli has a wider ecological range than its sexually reproducing progenitors and has colonized areas of the Central Lesser Caucasus. This expansion suggests that parthenogenetic lizards may have certain advantages over their sexually reproducing counterparts in specific environments, potentially leading to competitive exclusion and a reduction in the range of the parental species.

Parthenogenesis in Birds and Its Limitations

Parthenogenesis in birds is a rare occurrence and has been observed in a few domesticated species, including turkeys, chickens, and pigeons. In these cases, the unfertilized eggs usually do not hatch or the embryos do not develop properly. However, in turkeys, parthenogenesis can occasionally produce viable offspring, which often exhibit reduced fertility and other abnormalities. Selective breeding can increase the frequency of parthenogenetic offspring in these birds. A remarkable instance of avian parthenogenesis was documented in 2021 when two unfertilized eggs from California condors hatched at the San Diego Zoo. This event was notable because it occurred despite the presence of males, highlighting the potential for parthenogenesis even in species where sexual reproduction is the norm.

Challenges and Advances in Mammalian Parthenogenesis

Mammalian parthenogenesis does not occur naturally and is challenging to achieve even in a laboratory setting due to genomic imprinting, where genes are expressed differently depending on their parent of origin. This often results in developmental issues. Nonetheless, scientific progress has been made, such as the production of a bi-maternal mouse in 2004 through the manipulation of imprinted genes, which resulted in offspring with extended lifespans. While these advances are significant, mammalian parthenogenesis is not considered a viable reproductive strategy due to the associated developmental problems. Instead, research is focused on the potential of parthenogenetic processes to create embryonic stem cells for therapeutic applications.

Human Parthenogenesis: Myth, Reality, and Medical Potential

Human parthenogenesis has never been scientifically verified to result in a viable pregnancy and is more commonly found in mythology and religious narratives, such as the Christian doctrine of the Virgin Birth. However, there have been instances of partial parthenogenesis in humans leading to chimeric individuals, who possess a combination of fertilized and unfertilized cells. In medical research, parthenogenesis has been used to create human stem cells, which could be valuable for generating genetically compatible tissues for transplantation. Scientists have also developed human stem cell lines that are HLA homozygous through parthenogenesis, which could potentially provide tissue derivatives that are less likely to be rejected by the immune system, benefiting a broader range of patients.