Ovarian development and modes of apoptosis during oogenesis in various castes of the termite Reticulitermes aculabialis

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Abstract

Reproductive division in termites is the most significant biological process that leads to the formation of caste-specific differences in tasks and status. However, little is known about the mechanism of reproductive division that underlies caste differentiation. In the present study, ovarian development and stage-specific apoptotic patterns are investigated during oogenesis in reproductive, worker and soldier termites Reticulitermes aculabialis Tsai & Hwang. The results show that the mean lengths of the ovaries of reproductives are two-fold longer compared with those of workers and six-fold longer compared with soldiers. By contrast to the reproductives, the process of oogenesis in the workers includes only the oogonium differentiation stage (stage I) and oocyte growth stage (stage II), and oogenesis in the soldiers stops at stage I. Vitelogenic oocytes (stage III) are absent from workers and soldiers. During stage II in the reproductives and workers, the layer of follicle cells has a thickness of 7.56 ± 0.52 and 2.81 ± 0.34 µm, respectively. In addition, there are significant differences in the number and size of the germ cells at the same stage in the various castes. The existence of two apoptotic patterns during oogenesis is demonstrated by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay. First, the majority of the cells showing apoptosis occur at stage I of oogenesis in reproductives, workers and soldiers. Second, DNA fragmentation is demonstrated by TUNEL staining of the follicle cell layers and oocytes at stage II in reproductives. Finally, the proliferation activity of follicle cells in the reproductives is observed by 5-bromo-2′-deoxy-uridine labelling. The level of oogenesis may explain the significant discrepancies in the reproductive capacity among the reproductives, soldiers and workers. These large discrepancies are controlled by apoptosis during early oogenesis.

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