Final Activity Report Summary - SQUIDMATE (Testing for post-copulatory female choice in a cephalopod)
Charles Darwin believed that two main mechanisms were involved in sexual selection (natural selection focussing on sexual traits): male-male competition and female mate choice. Male-male competition ('sperm competition') and female choice are also believed to occur after copulation (in the female's body) in systems where females mate with multiple males. To assess the occurrence of female choice after copulation (cryptic female choice), one should:
1) show non-random use of sperm in fertilising eggs; and
2) show that the variation in sperm use results from female mediated processes (active or passive).
Unambiguous tests of whether cryptic female choice is or is not occurring are needed to establish the generality of this process, and therefore its importance in evolution. The South African squid Loligo vulgaris reynaudii represents an interesting study species due to the characteristics of its reproductive biology. Fertilisation is external, occurring as eggs are extruded in a long gelatinous string (several hundred eggs) from the oviduct and passed to the female's arms to be placed on the seabed.
Eggs can be fertilised using sperm stored in different locations: from the sperm storage organ (near the mouth) and from sperm packets placed by mates near the oviduct or around the mouth of the female. On the spawning grounds males outnumber females many times and so competition among males is fierce. Victorious males then guard their female and mating takes place in a parallel position, whilst small uncompetitive males try to 'sneak' matings with paired females.
The aims of this study were to determine the incidence and pattern of multiple paternity within broods of this squid, and to test concepts of cryptic female choice and sperm competition by analysing the relationship between patterns in offspring paternity and sperm located within particular storage sites. Results revealed a complex pattern of fertilisation in this species, with some broods exhibiting high levels of multiple paternity and some males siring significantly more offspring within broods than other males.
Where multiple sires were involved one male sired the majority of offspring at one end of the egg string but sired a substantially smaller percentage at the other end, which confirms non-random use of sperm. These results suggest that females may 'switch' sperm source being used during fertilisation, but further analysis of the data will be required to test if the variation in sperm use is in fact female-mediated or a consequence of physical constraints on the fertilisation dynamics (which would lead to a conclusion of sperm competition being the major controlling factor).
This study illustrates an interesting example of the complexity of natural processes going on during fertilisation and provides a model system for further research into the forces of evolution shaping reproduction in marine organisms.
1) show non-random use of sperm in fertilising eggs; and
2) show that the variation in sperm use results from female mediated processes (active or passive).
Unambiguous tests of whether cryptic female choice is or is not occurring are needed to establish the generality of this process, and therefore its importance in evolution. The South African squid Loligo vulgaris reynaudii represents an interesting study species due to the characteristics of its reproductive biology. Fertilisation is external, occurring as eggs are extruded in a long gelatinous string (several hundred eggs) from the oviduct and passed to the female's arms to be placed on the seabed.
Eggs can be fertilised using sperm stored in different locations: from the sperm storage organ (near the mouth) and from sperm packets placed by mates near the oviduct or around the mouth of the female. On the spawning grounds males outnumber females many times and so competition among males is fierce. Victorious males then guard their female and mating takes place in a parallel position, whilst small uncompetitive males try to 'sneak' matings with paired females.
The aims of this study were to determine the incidence and pattern of multiple paternity within broods of this squid, and to test concepts of cryptic female choice and sperm competition by analysing the relationship between patterns in offspring paternity and sperm located within particular storage sites. Results revealed a complex pattern of fertilisation in this species, with some broods exhibiting high levels of multiple paternity and some males siring significantly more offspring within broods than other males.
Where multiple sires were involved one male sired the majority of offspring at one end of the egg string but sired a substantially smaller percentage at the other end, which confirms non-random use of sperm. These results suggest that females may 'switch' sperm source being used during fertilisation, but further analysis of the data will be required to test if the variation in sperm use is in fact female-mediated or a consequence of physical constraints on the fertilisation dynamics (which would lead to a conclusion of sperm competition being the major controlling factor).
This study illustrates an interesting example of the complexity of natural processes going on during fertilisation and provides a model system for further research into the forces of evolution shaping reproduction in marine organisms.