Browsing by Author "Kubota, Kohei"

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Elementary budget of stag beetle larvae associated with selective utilization of nitrogen in decaying wood
(2018-06-01)
Tanahashi, Masahiko
;
Ikeda, Hiroshi
;
Kubota, Kohei
;
應用化學系
;
Department of Applied Chemistry
Wood degradation by insects plays important roles for the forest matter cycling. Since wood is deficient in nitrogen compared to the insect body, wood-feeding insects need to assimilate the nitrogen selectively and discard an excess carbon. Such a stoichiometric imbalance between food and body will cause high metabolic cost; therefore, wood-feeding insects may somehow alleviate the stoichiometric imbalance. Here, we investigated the carbon and nitrogen budgets of the larvae of stag beetle, Dorcus rectus, which feed on decaying wood. Assimilation efficiency of ingested wood was 22%, and those values based on the carbon and nitrogen were 27 and 45%, respectively, suggesting the selective digestion of nitrogen in wood. Element-based gross growth efficiency was much higher for nitrogen (45%) than for carbon (3%). As a result, the larvae released 24% of the ingested carbon as volatile, whereas almost no gaseous exchange was observed for nitrogen. Moreover, solubility-based elementary analysis revealed that the larvae mainly utilized alkaline-soluble-water-insoluble fraction of wood, which is rich in nitrogen. Actually, the midgut of the larvae was highly alkaline (pH 10.3). Stag beetle larvae are known to exhibit coprophagy, and here we also confirmed that alkaline-soluble-water-insoluble nitrogen increased again from fresh feces to old feces in the field. Stable isotope analysis suggested the utilization of aerial nitrogen by larvae; however, its actual contribution is still disputable. Those results suggest that D. rectus larvae selectively utilize alkaline-soluble nitrogenous substrates by using their highly alkaline midgut, and perhaps associate with microbes that enhance the nitrogen recycling in feces.
Specificity and genetic diversity of xylose-fermenting Scheffersomyces yeasts associated with small blue stag beetles of the genus Platycerus in East Asia
(2017-01-01)
Tanahashi, Masahiko
;
Kim, Jong-Kuk
;
Watanabe, Kana
;
Fukatsu, Takema
;
Kubota, Kohei
Among insect-fungus relationships, xylose-fermenting Scheffersomyces yeasts are well known for their potential in utilizing wood hemicelluloses and their association with various wood-feeding insects. However, their specificity to host insects or strain-level diversity within host species has not been clearly elucidated. In the insect family Lucanidae, larvae usually feed on decaying wood, and adult females consistently possess a fungus-storage organ, called the mycangium, near the abdominal tip. Here the authors investigated host-symbiont relationships between Scheffersomyces yeast symbionts and small blue stag beetles of the genus Platycerus (Coleoptera: Lucanidae) in East Asia by using intergenic spacer (IGS) region as a genetic marker. All yeast strains isolated from the female mycangiumof three Platycerus species, P. hongwonpyoi from Korea and P. acuticollis and P. delicatulus from Japan, were allied to Scheffersomyces segobiensis based on the sequences of the nrDNA 26S and internal transcribed spacer (ITS), in which no sequence difference was observed among those strains. However, IGS regions showed clear genetic differentiation within the yeast symbionts of P. hongwonpyoi, as well as between those of Korean and Japanese Platycerus species. In the IGS sequences, nucleotide substitutions were mainly distributed in the whole stretch of IGS1 and the anterior half of IGS2, whereas nucleotide gaps were localized at IGS1 and the middle of IGS2. Despite the conserved association between the Platycerus beetles and the specific strains of S. segobiensis in East Asia, geophylogenetic divergence patterns of the yeast symbionts were not concordant with those of the insect hosts.