Sexual selection increases sperm competitiveness
and sperm lengthJoanne L Godwin ([email protected]), PI: Matthew JG Gage
School of Biological Sciences, University of East Anglia
Despite huge diversity in sperm cells1,2 the pattern of sexual selection on gamete trait divergence
and the adaptive significance of these traits, remains unclear3,4.
Sperm size is predicted to trade against number, which is a key component of sperm competitiveness,
and a driver within anisogamy of the maintenance of small sperm size5.
Experimental evolution of sexual selection
Model: flour beetle, Tribolium castaneum
• highly promiscuous
• females store sperm from multiple males
• good evidence for post-copulatory sexual selection
1) Pitnick, S., Hosken, D.J. & Birkhead, T.R. (2009) Sperm morphological diversity. In: Sperm Biology: An Evolutionary Perspective. Oxford: Elsevier. 2) Gage, M.J. (2012) Complex
sperm evolution. Proceedings of the National Academy of Sciences. 109 (12), pp. 4341–4342. 3) Snook, R. (2005) Sperm in competition: not playing by the numbers. Trends in
Ecology & Evolution. 20 (1), pp. 46–53. 4) Pizzari, T. & Parker, G.A. (2009) Sperm competition and sperm phenotype. In: Sperm Biology: An Evolutionary Perspective. Oxford:
Elsevier. 5) Parker, G.A. (1982) Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes. Journal of Theoretical Biology. 96 (2), pp. 281–294.
What next? Organisation and behaviour of
different males’ sperm within
the female reproductive tract.
Measures of female
reproductive tract
divergence
Female-biased
‘weak’
sexual selection
low male-male/
sperm competition
& female choice
Male-biased
‘strong’
sexual selection
intense male-male/
sperm competition
& high opportunity
for female choice
versus balanced Ne
What have we found?
Significantly greater share
of paternity (P2) (p < 0.01)
for males with a history of
strong sexual selection.
Significantly longer sperm
(p < 0.01) in males
with a history of
strong sexual selection
No difference in
sperm length variance
between treatments.
Highly repeatable
measuring
technique.
Sexual selection
Anisogamy
leads to …
intra-sexual
competition
and …
inter-sexual
choice …
both pre- and
post-copulatory...
which maintains anisogamy.
>75 generations
:
10 : 90
FB
:
10 : 90
FC
:
10 : 90
FA
:
90 : 10
MB
:
90 : 10
MC
:
90 : 10
MA
