Olsson Lab

Mate Choice, Sperm Competition, and Cryptic Female Choice

How and why do animals choose a particular mate? The answers to this question can have dramatic impacts on how a species evolves. Mating between closely related individuals may lead to reduced offspring fitness due to inbreeding depression. This type of selective force could lead species to evolve mechanisms to identify genetically compatible mates. Our previous work on sand lizards (below left) has suggested strong genetic determinants of MHC-related pre- and post-copulatory mechanisms.

Females may also be able to choose a mate based on visual displays that may be a signal representing the quality of their genes. A male’s colourful visual display used to attract a mate is often costly in terms of predation and immunological stress resulting from high levels of testosterone. If this colour or behavioural display has a genetic basis it may be an honest signal as to the quality of that male’s genes (above right). Thus the offspring from a female would benefit from her ability to choose a mate with a bright visual display. We are investigating this idea in both the sand lizard where males have bright green side patches, and painted dragons males which have bright red, orange or yellow heads and yellow bibs.

In polyandrous species (i.e. when females mate with multiple males) fertilization can be biased by the relatedness of the parents, possibly due to recognition at the membrane molecule level. This implies that when females mate with several males prior to fertilization, within the oviduct selection is occurring for the sperm that is genetically suitable for the offspring. Similarly, with species that have external fertilization, it is likely that sperm from many males may compete for fertilization. To test ideas on selection at the gametic membrane it is necessary to bypass this barrier. This can be accomplished using techniques such as intracytoplasmic sperm injection (ICSI). We are using this technique on the Peron’s tree frog to address questions on assortative fertilization along with inbreeding and outbreeding depression.

DNA markers (microsatellites) are key to addressing many of these questions on mate choice and assortative fertilizations. By genotyping the mother and all the potential fathers we can confidently determine the paternity of the offspring, and thus identify the successful males in the populations and experimental trials.