Tokorhabditis tauri n. sp. and T. atripennis n. sp. (Rhabditida: Rhabditidae), isolated from Onthophagus dung beetles (Coleoptera: Scarabaeidae) from the Eastern USA and Japan

Sexually transmitted mutualist nematodes shape host growth across dung beetle species

Many symbionts are sexually transmitted and impact their host's development, ecology, and evolution. While the significance of symbionts that cause sexually transmitted diseases (STDs) is relatively well understood, the prevalence and potential significance of the sexual transmission of mutualists remain elusive. Here, we study the effects of sexually transmitted mutualist nematodes on their dung beetle hosts. Symbiotic Diplogastrellus monhysteroides nematodes are present on the genitalia of male and female Onthophagus beetles and are horizontally transmitted during mating and vertically passed on to offspring during oviposition. A previous study indicates that the presence of nematodes benefits larval development and life history in a single host species, Onthophagus taurus. However, Diplogastrellus nematodes can be found in association with a variety of beetle species. Here, we replicate these previous experiments, assess whether the beneficial effects extend to other host species, and test whether nematode-mediated effects differ between male and female host beetles. Rearing three relatively distantly related dung beetle species with and without nematodes, we find that the presence of nematodes benefits body size, but not development time or survival across all three species. Likewise, we found no difference in the benefit of nematodes to male compared to female beetles. These findings highlight the role of sexually transmitted mutualists in the evolution and ecology of dung beetles.

Geographical distribution and phoretic associations of the viviparous nematode Tokorhabditis atripennis with Onthophagus dung beetles in Japan

Viviparity is generally considered to be rare in animals. In nematodes, only six species of Rhabditida are viviparous. Five of these species have been identified in association with Onthophagus dung beetles, with Tokorhabditis atripennis being repeatedly isolated from the dung beetle Onthophagus atripennis in Japan. T. atripennis is easy to culture in a laboratory setting, and its host, O. atripennis, is distributed all over Japan. Therefore, T. atripennis is an ideal candidate for ecological and evolutionary studies on viviparity. However, the extent of their distribution and relationship with dung beetles, as well as habitats, remain unclear. In the present study, we conducted field surveys and successfully isolated 27 strains of viviparous nematodes associated with tunneler dung beetles from various regions of Japan, all of which were identified as T. atripennis. T. atripennis exhibited a strong association with Onthophagus dung beetles, especially O. apicetinctus and O. atripennis. And it was predominantly found in specific anatomical locations on the beetle bodies, such as the 'groove between pronotum and elytron' and the 'back of the wings'. Our findings suggest that Onthophagus species are the primary hosts for T. atripennis, and T. atripennis exhibits a close relationship with the living environments of tunneler beetles. This association may play a significant role in the evolution of viviparity in nematodes.

The developmental and structural uniqueness of the embryo of the extremophile viviparous nematode, Tokorhabditis tufae

Viviparity, a reproductive form that supplies nutrients to the embryo during gestation, has repeatedly and independently occurred in multiple lineages of animals. During the convergent evolution of viviparity, various modifications of development, structure, and physiology emerged. A new species of nematode, Tokorhabditis tufae, was discovered in the alkaline, hypersaline, and arsenic-rich environment of Mono lake. Its reproductive form is viviparity because it is obligately live-bearing and the embryo increases in size during development. However, the magnitude of the increase in size and nutrient provisioning are unclear. We measured egg and embryo sizes at three developmental stages in T. tufae. Eggs and embryos of T. tufae at the threefold stage were respectively 2.6- and 3.6-fold larger than at the single-cell stage. We then obtained T. tufae embryos at the single-cell, lima bean, and threefold developmental stages and investigated the egg hatching frequency at three different concentrations of egg salt buffer. Removal of embryos from the uterus halted embryonic development at the single-cell and lima bean stages in T. tufae irrespective of the solution used for incubation, indicating the provision of nutrients within the uterus. Ultrastructural and permeability evaluation showed that the permeability barrier did not form during embryonic development, resulting in increased molecular permeability. This high permeability caused by the absence of the permeability barrier likely enables supply of nutrients from the mother. The structural and physiological modifications in T. tufae are like those in other viviparous animals. We conclude that T. tufae is a viviparous rather than an ovoviviparous nematode. T. tufae will facilitate investigation of the evolution of viviparity in animals.

Trioecy is maintained as a time-stable mating system in the pink sea urchin Toxopneustes roseus from the Mexican Pacific

Trioecy is a sexual system that consists of the co-occurrence of females, males and hermaphrodites in a population and is common in plants; however, in animals it is uncommon and poorly understood. In echinoderms, trioecy had never been recorded until now. Frequencies of females, males, and hermaphrodites were evaluated and gametogenic development was histologically characterized in a population of Toxopneustes roseus inhabiting the Mexican Pacific. Trioecy in this population is functional and temporally stable, since the three sexes coexisted in each sampling month. The hermaphrodites presented similar gametogenic development as the females and males and participated during the spawning season, contributing to the population's reproductive process. Trioecy is considered an evolutionarily transitory state, and it is extremely difficult to explain its presence in a species. We hypothesize that continuous ocean warming represents a threat to the survival of this population of T. roseus, since its early developmental stages, which represent a population bottleneck, are more vulnerable to high temperatures than other sea urchins inhabiting the area, while its population density is significantly lower. These conditions generate a strongly stressed environment, which is the determining factor that maintains the stability of trioecy in the species in which it has been studied.