Aedes. albopictus (Skuse, 1895) is a vector of several arboviruses [1][2], including dengue and chikungunya [3].

This mosquito species has expanded its range considerably in recent years, threatening temperate regions that once had no risk of transmission [4] thanks to its tolerance to cold temperatures.

To control Ae. albopictus, several tools for mosquito population suppression were set up and involve the release of reared male mosquitoes in the lab and altered competing for and successfully inseminating wild females. Thus, they interfere and modify their reproduction [5].  It is the case of the sterile insect technique (SIT) which deploys males sterilized by radiation [6]. Because most females only mate once [7], [8], those females that mate to a SIT male experience reproductive failure [9] despite a few percentages of Ae. albopictus and Ae. aegypti are polygamous in nature and mate with and produce progeny from multiple males [7], [10]–[12].

This technique has the advantage of reducing adult mosquito populations while also producing larvae that compete for resources with wild larvae.

Nevertheless, results of modified male mosquito releases have been mixed [13], and failures could be due to factors of male biology which should be considered to improve the design of these release technique in the future.

While some research teams have studied different factors such as male vigor, insemination capacity, mating compatibility, and survival [14]–[16], the team of the department of entomology from Cornell University (USA) has recently examined the influence of age and body size in Ae. albopictus male mosquitoes on another parameter: the produced sperm and spermatozoid quantity [17]. Their results are published in the Journal of Medical Entomology.

From lab colonies of Ae. albopictus, researchers manipulated larval density to produce two. different adult size classes (large and small).

In total, they transferred 75 larvae for the large size class and 750 for the small size class, to mass rearing trays with different feeding regimes according to treatment

Then, males were separated and the length of their wings was measured. The average wings length was 2,48mm for the large males and 2,11mm pour the small males.

Dissections of their reproductive system were made at 1, 5, 10 and 20-day post-eclosion (dpe) for each size class in order to quantify the production of sperm, and therefore, the spermatozoids production.

Results showed that age and body size influence the sperm quantity produced by these male mosquitoes.

Indeed, large males produced significantly more sperm and spermatozoids than small males at each age up to 10 dpe.

Likewise, males produced more sperm as they aged, whatever their size.

However, the sperm quantity of small males began to stagnate from the age of 10 dpe while large males continued to produce sperm beyond this age, but the spermatozoid quantity wasn’t evaluated beyond 10 dpe.

These results suggest a divergence in resource availability or allocation, and large Ae. albopictus males would be able to invest more in sex than small males. 

These results remind the similar age and size dependent pattern existing in Ae. aegypti [18], despite this pattern presents a key difference: Ae. aegypti males’ sperm count peaked at 10 dpe, regardless of size.

This may reflect differences in the biology and/or in the larval diets of these two species.

This study shows that larval density and diet affect the spermatozoid quantity produced by Ae. albopictus males.

Given the fact that late larval and pupal stages can be critical periods for the spermatogenesis [19] and that resources obtained as larvae likely limit males’ lifetime potential sperm production, it is possible that large males were able to store more nutritional reserves from their larval diet than small males because of their lower larval density.

Indeed, an important larval density increases the competition for the resources access.

It explains that large males were able to continue to invest in the sperm production until the adult age, as the competition for the resources access was reduced during their larval period.  

Moreover, the production and the transfer of an excess sperm and spermatozoids quantity in the storage organs of some polygamous female mosquitoes during the mating, may be an advantage for males. Indeed, it would reduce the likelihood of sharing paternity with a second mate.

In contrast, abundant sperm may not directly increase the number of females that a male can sterilize in a modified male strategy such as the SIT.

It would have been interesting to study the viability of sperm maintained in old males as well as the spermatozoid quantity produced by males in natural conditions of mating and cohabitation with females, in order to observe if this production is modified or not.

Anyway, because spermatogenesis continues into old age, even in the absence of mating, this study reveals how old males are well equipped and able to allocate nutritional reserves to reproductive efforts.

Therefore, sperm capacity is a straight-forward means of quantifying male reproductive investment and may be used to fine-tune rearing protocols for male releases.

Thanks for reading!

And don’t forget: Fight Malaria!

 

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Bibliography

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