論文 - 下地　博之

Expansion of presoldier cuticle contributes to head elongation during soldier differentiation in termites

Yasuhiro Sugime, Kota Ogawa, Dai Watanabe, Hiroyuki Shimoji, Shigeyuki Koshikawa, Toru Miura

Science of Nature ( SPRINGER HEIDELBERG )  102 ( 11-12 ) 71 - 71   2015年12月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

　概要を見る

In termites, the soldier caste possesses morphological features suitable for colony defence, despite some exceptions. Soldiers are differentiated via two moultings through a presoldier stage with dramatic morphogenesis. While a number of morphological modifications are known to occur during the presoldier moult, growth and morphogenesis seem to continue even after the moult. The present study, using the damp-wood termite Hodotermopsis sjostedti, carried out morphological and histological investigations on the developmental processes during the presoldier stage that is artificially induced by the application of a juvenile hormone analogue. Measurements of five body parameters indicated that head length significantly increased during the 14-day period after the presoldier moult, while it did not increase subsequently to the stationary moult (pseudergate moult as control). Histological observations also showed that the cuticular development played a role in the presoldier head elongation, suggesting that the soft and flexible presoldier cuticle contributed to the soldier morphogenesis in termites.

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Social dominance and reproductive differentiation mediated by dopaminergic signaling in a queenless ant.

Yasukazu Okada, Ken Sasaki, Satoshi Miyazaki, Hiroyuki Shimoji, Kazuki Tsuji, Toru Miura

The Journal of experimental biology   218 ( Pt 7 ) 1091 - 8   2015年04月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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In social Hymenoptera with no morphological caste, a dominant female becomes an egg layer, whereas subordinates become sterile helpers. The physiological mechanism that links dominance rank and fecundity is an essential part of the emergence of sterile females, which reflects the primitive phase of eusociality. Recent studies suggest that brain biogenic amines are correlated with the ranks in dominance hierarchy. However, the actual causality between aminergic systems and phenotype (i.e. fecundity and aggressiveness) is largely unknown due to the pleiotropic functions of amines (e.g. age-dependent polyethism) and the scarcity of manipulation experiments. To clarify the causality among dominance ranks, amine levels and phenotypes, we examined the dynamics of the aminergic system during the ontogeny of dominance hierarchy in the queenless ant Diacamma sp., which undergoes rapid physiological differentiation based on dominance interactions. Brain dopamine levels differed between dominants and subordinates at day 7 after eclosion, although they did not differ at day 1, reflecting fecundity but not aggressiveness. Topical applications of dopamine to the subordinate workers induced oocyte growth but did not induce aggressiveness, suggesting the gonadotropic effect of dopamine. Additionally, dopamine receptor transcripts (dopr1 and dopr2) were elevated in the gaster fat body of dominant females, suggesting that the fat body is a potential target of neurohormonal dopamine. Based on this evidence, we suggest that brain dopamine levels are elevated in dominants as a result of hierarchy formation, and differences in dopamine levels cause the reproductive differentiation, probably via stimulation of the fat body.

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Global network structure of dominance hierarchy of ant workers.

Hiroyuki Shimoji, Masato S Abe, Kazuki Tsuji, Naoki Masuda

Journal of the Royal Society, Interface   11 ( 99 )   2014年10月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Dominance hierarchy among animals is widespread in various species and believed to serve to regulate resource allocation within an animal group. Unlike small groups, however, detection and quantification of linear hierarchy in large groups of animals are a difficult task. Here, we analyse aggression-based dominance hierarchies formed by worker ants in Diacamma sp. as large directed networks. We show that the observed dominance networks are perfect or approximate directed acyclic graphs, which are consistent with perfect linear hierarchy. The observed networks are also sparse and random but significantly different from networks generated through thinning of the perfect linear tournament (i.e. all individuals are linearly ranked and dominance relationship exists between every pair of individuals). These results pertain to global structure of the networks, which contrasts with the previous studies inspecting frequencies of different types of triads. In addition, the distribution of the out-degree (i.e. number of workers that the focal worker attacks), not in-degree (i.e. number of workers that attack the focal worker), of each observed network is right-skewed. Those having excessively large out-degrees are located near the top, but not the top, of the hierarchy. We also discuss evolutionary implications of the discovered properties of dominance networks.

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Egg discrimination by workers in Diacamma sp. from Japan

H. Shimoji, Y. Fujiki, R. Yamaoka, K. Tsuji

Insectes Sociaux ( Springer Science and Business Media LLC )  59 ( 2 ) 201 - 206   2012年05月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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In eusocial Hymenoptera, workers usually cannot mate but can lay male-destined haploid eggs. In contrast, in many species, worker reproduction is regulated by means of worker policing, which consists of two behavioral categories: aggression towards workers that have developed ovaries, and the destruction of worker-laid eggs. We focused on the latter behavior in the ant Diacamma sp. from Japan, in which a mated worker (gamergate) serves as the functional queen in each colony. An experiment in which we introduced both worker-laid eggs and gamergate-laid eggs from foreign colonies showed that workers preferentially destroyed the worker-laid eggs. This indicates that workers can discriminate worker-laid eggs from gamergate-laid eggs. In contrast, gamergates destroyed all introduced eggs that they discovered. Our chemical analysis of egg surface substances revealed that the worker-laid eggs clearly differed from gamergate-laid eggs in their surface hydrocarbons (SHCs), with the former containing more long-chain components. However, a bioassay to test whether specific components of the SHCs trigger the observed discrimination behavior by workers remains to be done. © International Union for the Study of Social Insects (IUSSI) 2011.

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Paternally inherited alleles in male body parts of an ant (Diacamma sp.) sex mosaic: Implication for androgenetic male production in the Hymenoptera

S. Dobata, H. Shimoji, H. Ohnishi, E. Hasegawa, K. Tsuji

Insectes Sociaux ( Springer Science and Business Media LLC )  59 ( 1 ) 55 - 59   2012年02月 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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Sex mosaicism, also called gynandromorphism, refers to an accidental phenomenon in dioecious organisms (mainly animals) in which an individual phenotype includes both female and male characteristics. Despite the rarity of this phenomenon, elucidating the mechanisms of naturally occurring sex mosaicism should deepen our understanding of diverse sex determination and differentiation systems in nature. We report the results of a genetic study of a sex mosaic individual of the ant Diacamma sp. from Japan's Okinawa Island. Parentage analysis using microsatellite markers revealed that the female and male parts of the sex mosaic showed different inheritance patterns: female parts had alleles consistent with their biparental inheritance, whereas most of the male parts had alleles consistent with their paternal inheritance (i. e., androgenesis). We discuss plausible cytogenetic mechanisms that gave rise to the male parts of this individual: polyspermy and the subsequent independent cleavage by a surplus sperm pronucleus, and maternal genome elimination after fertilization of an ovule. Moreover, we hypothesize that the androgenetically produced males found in some Hymenoptera might share the same underlying cytogenetic mechanism with hymenopteran sex mosaicism. © 2011 International Union for the Study of Social Insects (IUSSI).

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Do eggs convey information on gamergate presence in Diacamma sp. from Japan?

H. Shimoji, Y. Fujiki, K. Tsuji, R. Yamaoka

Insectes Sociaux ( Springer Science and Business Media LLC )  58 ( 1 ) 3 - 7   2011年 [ 査読有り ]

掲載種別: 研究論文（学術雑誌）

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In eusocial Hymenoptera, information on the presence of a queen is considered to play an important role in regulating worker reproduction. We examined the mode of transmission of this putative queen information to workers. Previous studies have reported that, in Diacamma sp. from Japan, unmated helper workers perceive the presence of a gamergate (functional queen) only through direct physical contact with her. In contrary to this conclusion, we showed, by an experiment that exposed workers either to the gamergate or to the eggs laid by the gamergate, that the presence of eggs laid by the gamergate to some extent suppressed the workers' dominance interaction, a behavior that is associated with self-reproduction by unmated workers. We discuss the relative importance of direct and indirect transmission of gamergate-presence information under natural conditions in this ant. © 2010 International Union for the Study of Social Insects (IUSSI).

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