Thursday, January 20, 2011

Lazy Sperms Check Inbreeding

Inbreeding (mating between relatives) is costly, primarily as a consequence of the expression of deleterious recessive alleles[1,2] from a overtly constricted gene-pool.
Some instances of adverse effects of inbreeding: The dwindling numbers of cheetahs has been attributed to a genetic bottleneck caused by heavy inbreeding [3], and in humans, appearance of several harmful traits (eg. Haemophilia in european royal families, 'reproductive wastage' in a population of Dammam, Saudi Arabia [4], Ellis-van Creveld  disease in Amish settlements [5], decreased fertility rates in Hutterites [6]  etc.) has been attributed to unabated consanguineous relationships.

Researchers have long suspected that polyandry - females taking multiple mates - evolved in some species as a strategy to reduce breeding with relatives, and therefore as a means to reduce the negative fitness consequences of mating with genetically related males.
Evidence suggests that the genetic relatedness (or genetic similarity) between mating partners is associated with competitive fertilization success [7]. In externally fertilizing fishes, it has been shown that the ovarian fluid (OF) released by the females with their eggs during spawning affects sperm swimming velocity [8], and this effect is influenced by the identity (genotype) of the interacting male and female [9]. Furthermore, there is ample evidence that sperm kinematic parameters are important determinants of sperm competition success [10].

Clelia Gasparini and Andrea Pilastro from the University of Padova, Italy, investigated female preference for unrelated mates in the guppy (Poecilia reticulata), an internally fertilizing species of fish, in which the females mate multiply.
Their aim was to experimentally demonstrate whether the selection bias which would determine mechanisms reducing fertilizations by genetically related mates was operating at the gametic (sperm and ova) level.

                                                  Guppies (source)
Reasons why Guppies were used as a model system:
1. ideal model to study post-copulatory mechanisms of inbreeding avoidance [11].
2. inbreeding has deleterious fitness consequences in this species [12].
3. males can undermine female choice by means of gonopodial thrusting [13], a coercive mating tactic that allows males to forcibly inseminate females.
4. Paternity analysis in natural populations has revealed a paternity skew towards unrelated males [14], suggesting that mechanisms reducing fertilizations by genetically related mates do operate in this species.

a. Paired sperm competition test in which sperms were artificially inseminated two unrelated females. The difference in paternity success across females in relation to the difference in genetic relatedness among mates was later analysed.
b. Determination of whether insemination from related males would result in a reduced brood size, via differential fertilization success or embryo viability, by comparing brood size of females that were artificially inseminated with the sperm from either a brother or an unrelated male.
c. Finally, exploration of physiological mechanisms by which females mediate inbreeding avoidance.

Critical Observations:
To confirm that polyandrous female guppies do give an edge to sperm from non-related males, Andrea Pilastro and Clelia Gasparini at the University of Padova in Italy performed artificial insemination of 28 virgin females with sperm from either an unrelated or related male.
* Related males fertilized 10 percent fewer eggs than unrelated males.

To then investigate the source of the advantage [expt. 'c'], the team looked at the interaction between sperm and OF.
In an in vitro computer assisted sperm analysis (commonly called CASA) measurement aimed at measuring critical kinematic parameters of sperm movement.
* A male guppy's sperm-velocity is 5-10 percent slower in the OF of his sister than that of an unrelated female.
In other words, it demonstrates that genetic relatedness influences the effect of OF on sperm performance by increasing the swimming velocity of sperm from unrelated males in the guppy. 

This throws up an important point, that there now exists an interesting coincidence between the effect measured in sperm velocity and that consequently found in paternity.
Future work / Possibilites:
How the guppies' genetics influence OF-sperm interaction is still unknown, but Pilastro suspects the involvement of signaling peptides/receptors etc. present on the sperm-surface.
Although it might be beyond the scope of this  investigation, but an 'In-vitro-fertilization' expt. (in the absense of 'OF') could have ruled out inherent inequalities in fertilization-capabilites of sperms from brothers and non-relative donors.
Although this may not be the only explanation for biased paternity, but it's an interesting paper since it has shown experimental evidence for this kind of selection for the first time. Indeed, it could also be a common mode of sexual selection among other species as well.


The paper: Clelia Gasparini and Andrea Pilastro, 2010. Cryptic female preference for genetically unrelated males is mediated by ovarian fluid in the guppy. Proceedings of the Royal Society. doi:10.1098/rspb.2010.2369

1. Thornhill, N.W. 1993 The natural history of inbreeding and outbreeding: theoretical and empirical perspectives. Chicago, IL: Chicago University Press.
2. Charlesworth, B. & Charlesworth, D. 1999 The genetic basis of inbreeding depression. Genet. Res. 74, 329–340
3. M Menotti-Raymond and S J O'Brien, 1993. Dating the genetic bottleneck of the African cheetah.  Proc Natl Acad Sci, 90(8): 3172–3176.
4. Al-Abdulkareem, A. and Ballal, S. 1998. Consanguineous Marriages in an Urban Area of Saudi Arabia: Rates and Adverse Health Effects on Offspring. Journal of Community Health, Vol. 23, No. 1, pp 75-83.
5. Dorsten, L., Hotchkiss, L., and King, T. 1999. The Effect of Inbreeding on Early Childhood Mortality: Twelve Generations of an Amish Settlement. Demography. Vol. 36. No. 2. pp. 263-271.
6. Ober, C., Hyslop, T., and Hauck, W. 1999. Inbreeding Effects on Fertility in Humans: Evidence For Reproductive Compensation. American Journal of Human Genetics. Vol. 64. pp 225-231.
7. Olsson, M., Shine, R., Madsen, T., Gullberg, A. & Tegelstrom, H. 1996 Sperm selection by females. Nature, 383, 585–585.
8. Urbach, D., Folstad, I. & Rudolfsen, G. 2005 Effects of ovarian fluid on sperm velocity in Arctic charr (Salvelinus alpinus). Behav. Ecol. Sociobiol. 57, 438–444.
9. Rosengrave, P., Gemmell,N. J.,Metcalf, V.,McBride, K.& Montgomerie, R. 2008 A mechanism for cryptic female choice in chinook salmon. Behav. Ecol. 19, 1179–1185.
10. Gage,M. J. G., Macfarlane, C. P., Yeates, S.,Ward, R. G., Searle, J. B. & Parker, G. A. 2004 Spermatozoal traits and sperm competition in Atlantic salmon: relative sperm velocity is the primary determinant of fertilization success. Current Biology. 14, 44–47.
11. Magurran, A. E. 2005 Evolutionary ecology: the Trinidadian guppy. Oxford, UK: Oxford University Press.
12. Zajitschek, S. R. K., Lindholm, A. K., Evans, J. P. & Brooks, R. C. 2009 Experimental evidence that high levels of inbreeding depress sperm competitiveness. J. Evolutionary Biology. 22, 1338–1345.
13. Pilastro, A. & Bisazza, A. 1999 Insemination efficiency of two alternative male mating tactics in the guppy (Poecilia reticulata). Proc. R. Soc. Lond. B 266, 1887–1891.
14. Johnson, A. M., Chappell, G., Price, A. C., Rodd, F. H., Olendorf, R. & Hughes, K. A. 2010 Inbreeding depression and inbreeding avoidance in a natural population of guppies (Poecilia reticulata). Ethology 116, 448–457.

* Slow sperm prevent inbreeding [story in 'The Scientist'] here.

* Insightful discussion on 'Inbreeding In Humans' [here].

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