Understanding sperm-egg interactions using fish and insect models

Bemrose, James (2021) Understanding sperm-egg interactions using fish and insect models. Doctoral thesis, University of East Anglia.

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Abstract

Sperm-egg interactions operate at the end of the struggle to reproduce and therefore have a significant influence over reproductive fitness and gene flow. In this thesis I utilise the externally fertilising Atlantic salmon and internally fertilising T. castaneum as complementary model species to understand sperm-egg interactions. Atlantic salmon’s (Salmo salar) external fertilisation system allows control and paired in vitro fertilisation experiments to reveal drivers and mechanisms of sperm-egg interactions. Tribolium castaneum flour beetles also enable tight control and generous replication, plus a generation time that enables experimental evolution. These systems and approaches were used to investigate both applied and fundamental questions about reproduction, fertilisation and gametic interactions in both wild and farm salmon. In Atlantic salmon, aquaculture and conservation hatcheries employ an artificial fertilisation process to reproduce offspring, which can sometimes lead to suboptimal hatch rates. Across a series of fertilisation tests, where potential factors within standard hatchery and farm gamete handling methods were investigated, we found wild Atlantic salmon hatch rate to be maximised when eggs were fertilised ‘dry’ (gametes are mixed together before the addition of river water) on the day of gamete stripping. In farm Atlantic salmon, we found no particular gamete handling factor to affect hatch rates, with females and eggs also able to tolerate external storage for extended periods of time. Overall, in both farm and wild fish, hatch rates were maximised when fertilised ‘dry’ with minimal post-ovulatory storage duration, and we found no evidence that any of these artificial gamete handling factors increased the risk of abnormal ploidy levels among offspring. However, if such storage is needed, eggs should be left within the coelomic cavity for up to 14 days, rather than exposing females to multiple checking and stripping events. Despite genetic and phenotypic divergence between wild and farm salmon through domestication, we found no barriers to hybridisation at the gamete level. Moreover, in our tests, farm Atlantic salmon sperm dominated paternity when in competition with sperm from wild males for eggs from wild females, suggesting that sperm quality has not been degraded through domestic selection for aquaculture. We also found no influence that ovarian fluid identity affected the outcomes of these in vitro fertilisation competitions. In T. castaneum, experimental evolution for 130 generations under high versus low opportunities for sexual selection and cryptic female choice did not reveal any evidence for divergence in female ability to choose the ‘right’ kind of sperm when inseminated with both conspecific and heterospecific sperm. A further experiment on these high and low sexual selection line females found no evidence that divergence in offspring production rates had evolved: across 100 days of oviposition, females from high and low sexual selection lines produced similar offspring number at similar rates when males were freely accessible (in contrast to previous findings that the lines differ in reproductive output under sperm limitation). In summary, these combined experiments add to our applied and basic knowledge of processed at the gamete level that can have important consequences for reproduction. Through looking at the gamete level in Atlantic salmon, I was able to determine measures to improve production and survival rates of the early life stages. In T. castaneum I focused on the influence of sexual selection intensity on female ability to assert cryptic female choice and manipulate offspring production rate, and I found no clear evidence for such postcopulatory choice, despite clear opportunities for its actions to have evolved.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Nicola Veasy
Date Deposited:	29 Nov 2022 14:08
Last Modified:	29 Nov 2022 14:08
URI:	https://ueaeprints.uea.ac.uk/id/eprint/89971
DOI:

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