Fertility and sperm studies in insects.
I consider sperm not only as cells consistuting an ejaculate but also as individuals, each having morphological and physiological characteristics.
The fertilizing capacities of individual males can be seen as a combination of both quantity and quality of spermatozoa.
Among parasitoid hymenoptera, some species show a reduced sperm count (some hundreds) associated to a strong fertlizing capability. It results in sperm having an individual paternity assurance dramatically higher than those of usual animals. In those species, males are issued from unfertilized eggs and only females result from the use of sperm by females.
We have conducted experimental lab work to count sperm at different stages of the reproductive process: in males before copulation, in ejaculates, and in females in the course of egg laying. It evidences that females which were not fully inseminated (by too young males for instance) have an insufficient sperm stock to complete their offspring sex ratio.
When males are stressed during their pupal development, either by cold, heat or chemicals, they have a reduced sperm stock. This results in subfertile males. Such males give rise to sperm constrained females, which could not produce optimal sex ratios.
Then, the relation between sperm quantity, quality and patterns of reproduction includes an ecological dimension that is linked to the conditions of the male's development.
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Bressac C. and Chevrier C. 1998. Offspring and sex ratio are independent of sperm management in Eupelmus orientalis females. Journal of Insect Physiology. 44 :351-359.
Damiens D., Bressac C., Brillard J.-P. and Chevrier C. 2002. Qualitative aspects of sperm stock in males and females from Eupelmus orientalis and Dinarmus basalis(Hymenoptera: Chalcidoidea) as revealed by dual fluorescence. Physiological Entomology. 27:97-102.
Lacoume S., Bressac C. and Chevrier C. 2009. Male hypofertility induced by Paraquat consumption in the non-target parasitoid Anisopteromalus calandrae (Hymenoptera: Pteromalidae). Biological control, 49:214–218.
King B.H. and Bressac C. 2010. No fitness consequence of experimentally induced polyandry in a monandrous wasp. Behaviour, 147:85-102.
Nguyen T.-M, Bressac C. and Chevrier C. 2013. Heat stress affects male reproduction in a parasitoid wasp. Journal of Insect Physiology, 59 :248-254.