Sexual selection increases sperm competitiveness

and sperm lengthJoanne L Godwin (j.godwin@uea.ac.uk), 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