Parthenogenesis is a form of asexual reproduction whereby females eggs develop into embryos without fertilization from a male. While this phenomenon is commonly observed in fish, reptiles, and plants, it is rare or unknown in birds such as the California condor.
As part of the captive breeding program of California condors, Oliver Ryder and his team developed a genetic database for all California condors such that breeding programs can work to maintain genetic variation in condor populations and prevent inbreeding, as described in an article published in October in Oxford Academic’s Journal of Heredity. Ryder and his team reviewed the genetic data of 911 California condors born in the captive breeding program. The team found two instances of parthenogenesis, one in 2001 and another in 2009. This is the first time reproduction of this kind has been observed in California condors.
California condors, like most animals, are diploid, meaning that they have two copies of each of their chromosomes, one inherited from the egg and the other from the sperm. In the case of parthenogenesis, there is no male to provide half of the child’s chromosomes, so both sets of genetic material come from the mother. It was this fact that led researchers to discover the cases of parthenogenesis: all 21 of the genetic markers compared were the same on both copies of the chromosomes. Researchers could identify a mother, but not a father.
Both offspring were male, which also supports the occurrence of parthenogenesis because this is the only viable combination of sex chromosomes that could both come only from a mother. An individual’s sex is determined by their combination of sex chromosomes, and for bird species such as California condors, males have ZZ sex chromosomes, and females have ZW. Thus, female eggs will either contain a Z or W chromosome, and the offspring from parthenogenesis will be either ZZ or WW. The Z chromosome contains necessary genes, so all viable offspring from parthenogenesis will be ZZ and therefore male.
These instances of parthenogenesis in California condors were unusual because they were in a captive breeding program, meaning that males were present with the females. In previous cases, parthenogenesis is known to occur only when a female cannot find a male to mate with. This may be because offspring resulting from parthenogenesis (parthenotes) do not have as much genetic variation as offspring resulting from sexual reproduction, and it is more likely that unfavorable traits may be passed down. However, while the two parthenotes both died at fairly young ages of 1.9 and 7.9 years old, their deaths weren’t explicitly related to parthenogenesis and their lack of genetic diversity.
More research is needed to determine if parthenotes can reproduce themselves and how parthenogenesis may factor into the genetic diversity and range expansion for California condors. However, pathogenesis could be beneficial for California condors in the long term.
This article was edited by Justin Adler and Emi Krishnamurthy.