Mechanisms and consequences of hybridisation between Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Diamond, Sian (2012) Mechanisms and consequences of hybridisation between Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). Doctoral thesis, University of East Anglia.

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Relatively little research has been done to investigate the way postcopulatory, prezygotic
mechanisms act to isolate species at the level of the gamete. This thesis uses the naturallyhybridising,
externally-fertilising system of Atlantic salmon, Salmo salar, and brown trout, S.
trutta, to investigate mechanisms of hybridisation through sperm-egg interactions, much of
which is poorly understood.
Salmon and trout experience conspecific sperm precedence during in vitro sperm
competition experiments, when sperm volumes and release times are equalised. This thesis
firstly aimed to explore the dynamics of gametic interactions underlying this reproductive
isolation. Manipulating the sperm entry time in interspecific sperm competitions significantly
influenced the observed conspecific sperm precedence. A 2 second delay to the entry of
conspecific sperm did not give hybridising males first-male sperm precedence, but neither did
they gain precedence with paternity being shared between males; suggesting a mechanism of
selection for conspecific sperm. Selection mechanisms were investigated through in vitro sperm
competitions where egg ovarian fluid type was manipulated. Results showed that conspecific
ovarian fluid allowed conspecific sperm significantly higher fertilisation success when
competing against heterospecific sperm, regardless of which species eggs were under
competition. This is the first evidence for cryptic female choice via a reproductive fluid in an
external fertiliser.
The second objective of my thesis was to investigate the potential consequences of
salmon-trout hybridisation for wild populations. This was achieved through comparing the early
life and reproductive fitness of hybrids and pure species. Both reciprocal hybrid crosses had
comparable early life fitness to pure species. Importantly however, neither reciprocal cross
exceeded pure juveniles for any fitness measures. This suggests the replacement of parental
species by hybrids is unlikely. Both hybrid crosses were capable of producing viable sperm and
able to fertilise over 50% of both salmon and trout eggs. Neither cross gained paternity success
when competing for trout eggs with conspecific males, while very low paternity was gained
under sperm competition with Atlantic salmon for salmon eggs. The main threat posed by
hybridisation to vulnerable salmon populations appears to come from wasted reproductive effort,
through the production of reproductively unfit hybrids. The implications of this are discussed.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Users 2593 not found.
Date Deposited: 15 May 2013 08:18
Last Modified: 15 May 2013 08:18

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