Invertebrate solutions for sensing gravity

In species with paternal genome elimination, males do not pass on their genes through their sons (they do not make it into their sons' sperm) and might therefore favour daughters. The same is true for the bacteria, because these are only transmitted through the cytoplasm in scale insect eggs (and so through the maternal line of their hosts). Females, however, benefit from both male and female offspring. Therefore, all three of them have an interest in controlling sex determination, which might have led to the large diversity of unusual genetic systems through evolutionary conflicts between female-expressed genes, male-expressed genes, and the symbiotic bacteria. Males do seem to get a bit of a rough deal, don't they? This also might have to do with their bacteria. As the endosymbionts are only transmitted through females there is no selection pressure for them to do their job that well in males. Even worse, it might be beneficial for the bacteria to kill males if this benefits their sisters, who carry the relatives of the bacteria. In order to avoid this, males appear to have been selected to depend less on their bacteria, which might explain their small size and short lifespan. There is even a scale insect where males don't have bacteria at all and do not feed. Instead they are fed by their mothers, who have evolved a placenta-like structure for this purpose. Does the host have any means of controlling their bacteria? Perhaps. Most scale insects have a specialized organ called the bacteriome which contains their endosymbionts; this structure can take up about one third of their body. Several very peculiar mechanisms for the formation of this organ have evolved in different scale insect families. In some, it is formed from modified gut cells, whilst in mealybugs and armoured scales it is formed by the fusion of the maternal polar bodies (formed during oogenesis) and one embryonic cell, thus forming a chimera of maternal and offspring tissue. In species from the family Putoidea, the bacteriome of an individual does not contain any of its own tissue but is completely formed from maternally-derived cells! Although many of these mechanisms seem puzzling, they have in common that they make sure it is difficult for the bacteria inside these organs to infer the sex of their host from the genetic make-up of the tissue around them, perhaps helping suppress spiteful behaviour against males. Do …