Histopathology of laboratory-reared Nothobranchius fishes: Mycobacterial infections versus neoplastic lesions.

Short-lived killifishes of the genus Nothobranchius Peters, 1868 (Cyprinodontiformes) are considered promising model organisms for biomedical research on ageing and tumorigenesis. We conducted histopathological analysis of 411 adult individuals from three Nothobranchius species to study details on spontaneous age-related neoplastic lesions. Light microscopy based on H&E and toluidine blue-stained sections revealed (a) non-proliferative liver changes with pronounced vacuolation of hepatocytes; (b) proliferation of kidney haemopoietic tissue contributing to excretory system damage; (c) proliferation of splenic mononuclear haemoblasts accompanied by reduced erythropoiesis; (d) proliferation of mononuclear cell aggregates in the liver parenchyma; and (e) rare occurrence of hepatocellular adenomas. Ziehl-Neelsen (ZN) staining revealed that the proliferative lesions are a host defence response to mycobacterial infections manifested by activation of the mononuclear phagocytic system and atypical granulomatous inflammatory reaction. 16S rRNA analysis identified three species of Mycobacterium in our samples. Our findings turn attention to lesions which mimic neoplasms by their gross appearance and question the light microscopic interpretation of lesions unless differential ZN staining is included. Beyond the limitations of our morphological approach, the intensity of mycobacterial infections is a challenging opportunity for research into the molecular-genetic background of the mononuclear phagocytic system reaction in Nothobranchius killifish.

Spontaneous adenocarcinoma of the gas gland in Nothobranchius fishes.

Nothobranchius fishes (Cyprinodontiformes), known for their genetically encoded extremely compressed lifespan, are considered an excellent vertebrate model for the research of aging. Unlike the rapid accumulation of data concerning their biology, ecology and genome, knowledge of their age-related diseases, including tumours, is still very limited. This Note reports spontaneous neoplastic lesions in the swim bladder gas glands of Nothobranchius furzeri, N. kadleci and N. orthonotus. Based on light and transmission electron microscopy, the neoplastic proliferation of gas gland cells was classified as adenocarcinoma. There was a concurrent proliferation of haemopoietic cells in the kidney interstitium in all individuals diagnosed with this type of primary neoplasia.

Repeated intraspecific divergence in life span and aging of African annual fishes along an aridity gradient.

Life span and aging are substantially modified by natural selection. Across species, higher extrinsic (environmentally related) mortality (and hence shorter life expectancy) selects for the evolution of more rapid aging. However, among populations within species, high extrinsic mortality can lead to extended life span and slower aging as a consequence of condition-dependent survival. Using within-species contrasts of eight natural populations of Nothobranchius fishes in common garden experiments, we demonstrate that populations originating from dry regions (with short life expectancy) had shorter intrinsic life spans and a greater increase in mortality with age, more pronounced cellular and physiological deterioration (oxidative damage, tumor load), and a faster decline in fertility than populations from wetter regions. This parallel intraspecific divergence in life span and aging was not associated with divergence in early life history (rapid growth, maturation) or pace-of-life syndrome (high metabolic rates, active behavior). Variability across four study species suggests that a combination of different aging and life-history traits conformed with or contradicted the predictions for each species. These findings demonstrate that variation in life span and functional decline among natural populations are linked, genetically underpinned, and can evolve relatively rapidly.