. The Biological bulletin. Biology; Zoology; Marine biology. 200 D. M. KRUEGER ET Figure 3. Transmission electron micrographs showing bacteria inside juvenile Solemya velum gills cells. (A) Section of 11-day-old gill with inclusions resembling gram-negative bacteria in nonciliated cells at the base of developing filaments. Many of these bacteria appear to be dividing (arrow). Scale bar = iim. (B) Longitudinal section through a single filament of a 64-day-old individual showing alternating symbiont- containing bacteriocytes and symbiont-free, goblet-shaped intercalary cells along the f
. The Biological bulletin. Biology; Zoology; Marine biology. 200 D. M. KRUEGER ET Figure 3. Transmission electron micrographs showing bacteria inside juvenile Solemya velum gills cells. (A) Section of 11-day-old gill with inclusions resembling gram-negative bacteria in nonciliated cells at the base of developing filaments. Many of these bacteria appear to be dividing (arrow). Scale bar = iim. (B) Longitudinal section through a single filament of a 64-day-old individual showing alternating symbiont- containing bacteriocytes and symbiont-free, goblet-shaped intercalary cells along the filament proximal to the ciliated edge. The bacteria are concentrated near the outer edge of the host cell, have gram-negative cell walls, and are contained within individual membrane-bounded vacuoles (inset). Scale bars = 1 ^m. i = intercalary cell, n = bacteriocyte nucleus, s = symbiont, mv = microvilli.' this result could be caused by technical inconsistencies in the assay, an alternative explanation is that these gonad samples had too few bacteria to yield a PCR product. This could be the case if, for example, the clams had recently spawned. Overall, the data suggest that female S. velum maintain a continually maturing population of oocytes with a seed population of symbionts. This conclusion is consistent with the observed intimacy of the adult sym- biosis and the hypothesis that S. velum symbionts are transmitted vertically from host generation to generation. Cary (1994) used a similar approach of screening host tissues with PCR and symbiont-specific primers in his study of symbiont transmission in the congener species S. reidi. Transovarial symbiont transfer was also hypoth- esized for S. reidi on the basis of its nutritional dependence on the chemoautotrophic symbionts in its gills (Fisher, 1990)—adult S. reidi completely lack both mouth and gut! (Reid, 1980). Positive PCR amplification of symbiont genes from DNA extracted from host gills, ovaries, eggs, and larvae ind
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