Rapid determination of sperm number in ant queens by flow cytometry

In social Hymenoptera, queens receive a given amount of sperm during a single or multiple inseminations once and for all. The amount of sperm stored at mating determines the maximum number of fertilized eggs queens can produce for the rest of their reproductive life. We propose flow cytometry (FCM) as a method to estimate the concentration of sperm cells, as well as their ploidy level, in queens' spermathecae. Our data, obtained from 5 ant species, show that FCM is precise, repeatable, easy to conduct and rapid. Estimates of variation of spermathecal content always remain below 10%, and samples can be analysed in less than 5 minutes. Flow cytometry appears as an excellent method for comparative analyses of sperm number within and between ant species.