About the Blog Author: Alexis Oetterer received her BSc in Biology from Truman State University and is currently a lab tech in Dr. Stacy Krueger-Hadfield’s lab at the University of Alabama at Birmingham. She will start grad school in the spring and is interested in studying life cycle and reproductive evolution and ecology.
Sex is costly: opportunities to find a mate can be rare and even if an organism is able to find one, the act of mating can be risky. Two ways to solve these problems are parthenogenesis and sperm storage, which allow for the reproduction of offspring without recent involvement of a mate. Parthenogenesis is the production of offspring without fertilization and a male gamete. Sperm storage allows for sperm to be stored in the female reproductive tract for long periods after mating, even for up to seven years in some species (Magnusson 1979)! Without genetic analyses, it’s difficult to determine which mode of reproduction is occurring since both parthenogenesis and sperm storage result in the production of offspring after isolation from mates. If offspring are produced through sperm storage, they have genetic contributions from both mother and father; if offspring are produced by parthenogenesis, they lack a paternal contribution.
Sperm storage is assumed to be widespread in reptiles and multiple mating is common. However, family-living tree skinks (Egernia striolata, see Figure 1) are socially monogamous, have less sexual conflict, and have lower rates of multiple paternity. Riley et al. (2021) explored whether egerniine skinks reproduce through parthenogenesis or sperm storage.
To test this, gravid female tree skinks were caught near Albury, NSW, Australia. After capture, the females were housed individually and were monitored until they gave birth. The first litters were born in the summer of 2014 and tail tissue was taken from the offspring for genetic analyses. After females gave birth, they were transferred to an outdoor enclosure with no access to males. The following summer, three females gave birth (two of which had given birth the previous year) and tail tissue was taken from the offspring.
Using genetic analyses, the authors quantified relatedness (paternity and sibship) within and between litters from 2014 and 2015 and reconstructed genotypes of the fathers. All litters in 2014 and 2015 consisted of full siblings within respective litters. Both litters from 2015 had different fathers than the 2014 litters and offspring were more heterozygous.
Based on kinship within and between litters, they concluded that the second set of litters were produced from sperm storage, which is the first time it has been documented in any monogamous reptile. Tree skinks live in family groups, so mating outside their social or spatial groups and storing sperm may help to avoid inbreeding. Extra-pair mating may also be a form of genetic bet-hedging, as the offspring that resulted from sperm storage were more heterozygous than those from recent mating. Further studies are needed to determine costs and benefits of sperm storage and extra-pair mating, how females may manipulate paternity, what role the reproductive tract structure plays in sperm storage and multiple paternity, and how widespread sperm storage is in social reptiles.