Both asymmetric mitotic segregation and cell-to-cell invasion are required for stable germline transmission of Wolbachia in filarial nematodes

Parasitic filarial nematodes that belong to the Onchocercidae family live in mutualism with Wolbachia endosymbionts. We developed whole-mount techniques to follow the segregation patterns of Wolbachia through the somatic and germline lineages of four filarial species. These studies reveal multiple evolutionarily conserved mechanisms that are required for Wolbachia localization to the germline. During the initial embryonic divisions, Wolbachia segregate asymmetrically such that they concentrate in the posteriorly localized P(2) blastomere, a precursor to the adult germline and hypodermal lineages. Surprisingly, in the next division they are excluded from the germline precursor lineage. Rather, they preferentially segregate to the C blastomere, a source of posterior hypodermal cells. Localization to the germline is accomplished by a distinct mechanism in which Wolbachia invade first the somatic gonadal cells close to the ovarian distal tip cell, the nematode stem cell niche, from the hypodermis. This tropism is associated with a cortical F-actin disruption, suggesting an active engulfment. Significantly, germline invasion occurs only in females, explaining the lack of Wolbachia in the male germline. Once in the syncytial environment of the ovaries, Wolbachia rely on the rachis to multiply and disperse into the germ cells. The utilization of cell-to-cell invasion for germline colonization may indicate an ancestral mode of horizontal transfer that preceded the acquisition of the mutualism.