Bacteriocyte population growth in Blattella germanica

The number of bacteriocytes with nucleus in the M or S phase was analysed in relation to their rate of increase, throughout two nymphal instars (N1 and N6) of Blattella germanica Linnaeus (Blattaria, Blattellidae). We treated the experimental specimens with colcemid in order to visualize C-metaphases, and with labelled thymidine in order to evidence DNA synthesis in these bacteria-carrying cells. In both young and old nymphs, the C-metaphases showed a similar trend: their average number was only 0.3 per 100 bacteriocytes counted throughout the entire instars. In the young nymphs, the number of bacteriocytes in the S phase was congruent with the frequencies of the C metaphases. Since the number of M or S bacteriocytes was not sufficient to account for the observed increase in number of these endosymbiotic cells during nymphal development, we hypothesize a mechanism of bacterial transmission from bacteriocytes to other fat body cells that may explain the numerical growth of the bacteriocyte population.