THE DISTRIBUTION OF CYTOPLASMIC BACTERIA IN THE EARLY DROSOPHILA EMBRYO IS MEDIATED BY ASTRAL MICROTUBULES

Maternally inherited cytoplasmic bacteria have occasionally been obser ved in embryos and adults of different strains of several Drosophila s pecies. While there is a considerable body of data on the relationship between bacteria and embryo viability, little is known about the beha vior of these bacteria during the early development of Drosophila. In eggs laid by infected Drosophila melanogaster females we showed that c ytoplasmic bacteria were initially concentrated in a thin cortical lay er and scattered in the yolk region. During the following syncytial bl astoderm mitoses the bacteria mainly accumulated towards the poles of the mitotic spindles, suggesting that astral microtubules play a role in localizing bacteria. This is supported by the observation that trea tment of the infected embryos with the microtubule-disrupting drug col chicine led to the partial dissociation of the bacteria from the spind le poles, whereas cytochalasin treatment left almost all the bacterial clusters intact. Moreover, bacteria were not found near the polar bod ies and yolk nuclei, which were without astral microtubules. In mitosi s-defective embryos, with centrosomes dissociated from the nuclei, the bacteria were concentrated in association with the isolated astral mi crotubules, and in cold-treated embryos, in which microtubules regrew from isolated centrosomes after recovering, the bacteria clustered aro und the newly formed asters. These observations, also supported by ele ctron microscope analysis, indicate a close relationship between cytop lasmic bacteria and astral microtubules, and suggest that the latter w ere able to build discrete cytoplasmic domains ensuring the proper dis tribution of cytoplasmic components during the blastoderm mitoses, des pite the lack of cell membranes.