Epixenosomes: Peculiar Epibionts of the Hypotrich Ciliate Euplotidium Itoi Defend Their Host Against Predators

ABSTRACT. Euplotidium itoi harbors on its dorsal surface peculiar episymbionts (referred to as epixenosomes) equipped with a complex extrusive apparatus. In the laboratory. E. itoi stocks without epixenosomes behave and reproduce like symbiotized stocks. the hypothesis that epixenosomes play a defensive role against predators was tested by comparing the behavior of Litonotus lamella when preying upon Euplotes crassus, E. itoi without epixenosomes. and E. itoi with epixenosomes. Litonotus discharges its toxicysts upon direct-cell-to cell contact, and paralyzes the three types of prey with the same efficiency. Nevertheless, Litonotus can ingest Euplotes, Euplotidium without epixenosomes, and to a certain extent, Euplotidium with epixenosomes whose ejecting capability has been inhibited. while it never eats Euplotidium with unaltered epixenosomes. In each prey-type, about 60% of the individuals attacked by Litonotus toxicyst discharge are able to recover their normal behavior once transferred into pure sea water. This percentage for E. itoi with epixenosomes that are never eaten by the predator corresponds to the probability of survival. This probability is lower for the other two prey-types in which the prey engulfed by the predator do not have the chance to recover. These data support the hypothesis and suggest the involvement of the epixenosome's ejecting apparatus in a defensive function.     

The chromosome make-up of mouse embryonic stem cells is predictive of somatic and germ cell chimaerism

Mouse pluripotent embryonic stem (ES) cells, once reintroduced into a mouse blastocyst, can contribute to the formation of all tissues, including the germline, of an organism referred to as a chimaeric. However, the reasons why this contribution often appears erratic are poorly understood. We have tested the notion that the chromosome make-up may be important in contributing both to somatic cell chimaerism and to germ line transmission. We found that the percentage of chimaerism of ES cell-embryo chimaeras, the absolute number of chimaeras and the ratio of chimaeras to total pups born all correlate closely with the percentage of euploid metaphases in the ES cell clones injected into the murine blastocyst. The majority of the ES cell clones that we tested, which were obtained from different gene targeting knockout experiments and harboured 50 to 100% euploid metaphases, did transmit to the germline; in contrast, none of the ES cell clones with more than 50% of chromosomally abnormal metaphases transmitted to the germline. Euploid ES cell clones cultured in vitro for more than 20 passages rapidly became severely aneuploid, and again this correlated closely with the percentage of chimaerism and with the number of ES cell--embryo chimaeras obtained per number of blastocysts injected. At the same time, the ability of these clones to contribute to the germline was lost when the proportion of euploid cells dropped below 50%. This study suggests that aneuploidy, rather than loss of totipotency, in ES cells, is the major cause of failure in obtaining contributions to all tissues of the adult chimaera, including the germline. Because euploidy is predictive of germline transmission, karyotype analysis is crucial and time/cost saving in any gene-targeting experiment     

Social status influences microhabitat selection: breeder and floater Eagle Owls Bubo bubo use different post sites

Social status can be reflected in many aspects of an individual’s behaviour and ecology, including habitat use and conspecific interactions. In territorial species where at least two social groups – breeding birds and non‐territorial floaters – are recognized, the diverse tasks associated with territorial ownership can lead territory holders to behave differently from the non‐territorial part of the population. Territory holders defend their breeding area and reproduce, whereas floating individuals are dispersing and lead a more transient life, during which they do not show any territorial behaviour even when settling in a more or less fixed area (known as the stop phase). As social interactions are based on visual and vocal cues, the use of specific sites for sending and/or receiving signals can be a crucial choice in an animal’s life. By analysing the post‐site selection of Eagle Owl Bubo bubo breeders and floaters during their nocturnal activity, we found that: (1) territory holders selected more visible and dominant posts than non‐territorial floaters; (2) the choice of posts made by floating individuals did not differ between the wandering and stop phases of dispersal; and (3) floating females intruded more frequently than floating males within a breeder’s home‐range. These findings highlight the fact that two social strategies are possible within the same species, depending on an individual’s social status and its related tasks. Breeders could take advantage of visible locations to declare their status as territory holders, whereas floaters could benefit from a more secretive life to wander unnoticed among occupied territories. This secretive life would help floaters to reduce the risks associated with conspecific aggression. Finally, the greater occurrence of floating females within breeders’ home‐ranges can be explained by the fact that female incursions in a breeder’s home‐range are less risky than male intrusions.