Archive for April, 2006

Daphnia pulex

Daphnia pulex

This organism belongs to Crustacea, Arthropoda. We are also working on the predator-induced polyphenism seen in some species of the genus Daphnia. They produce special structures such as horns or spines at the top of their heads.

In the case of our focal species Daphnia pulex, the chemical substances called ‘kiromone’ secreted from predator species, induces a few spines at the back of their heads. Those spines are called as ‘neckteeth’. If this structure exists on their heads, the predatatory mosquito larvae cannot feed them, because these spine stuck in their throat. We are studying on the developmental mechanisms that produce this structure.

In addition, they have interesting life cycle, similar to aphids. Most generations produce only female individuals parthenogenetically. But under bad conditions such as starvation, they produce male and female individuals which mate with each other and then produce diapausing eggs.

I think this type of animals like aphids and daphnids, potentially possess features that plastically change their phenotypes by controlling their embryonic and postembryonic development.

Caste determination in Diacamma

Diacamma sp.

Most of ant species possess several types of female individuals even in a single colony, namely reproductive individual (queen) and one or more types of helpers (workers). But some ant species exhibit only one type of female individuals, among which reproductives and helpers are determined behaviorally.

The above picture is Diacamma sp. that belongs to the subfamily Ponerinae. This species produces only a single type of female adults, that are wingless. Instead of wings, they possess vestigial wings called “gemmae”. They perform very peculiar behavior when a new adult emerges from cocoon.

If a new female individual is mutilated with her gemmae by other colony members (workers), she becomes workers. While, if a new female is not mutilated, she becomes reproductive, called “gamergate”. Thus, this vestigial wings function as “caste-determination switch”. This type of behaviorally-determined caste system provides us a key system for understanding behavioral basis of sociality seen in various social insects.

Juvenile hormone

juvenile hormone III

Juvenile hormone (JH), one of the important insect hormones, plays various important roles in the physiological regulations in insects. In our study on insect polyphenism, for example, JH triggers various morphological changes such as phase polyphenism in locusts and aphids, caste differentiation in social insects etc. Many of the insects use juvenile hormone III (JHIII), which is the simplest form of JH. JHI and JHII are also known, but only in Lepidopteran insects.

However, the mechanisms of reception of this hormone remains to be unsolved. No one knows the receptor for JH. Many works suggested that the threshold for JH must be the important regulatory mechanism of polyphenic traits.

Interestingly, various types of analogues for this molecule (JHA) can mimic the function. For example, in termites, we can induce soldier differentiation by the artificial applicaiton of JHA. We often use this method to investigate the developmental mechanism of soldier differentiation.

Arboreal termite nest

Nest of Nasutitermes takasagoensis

Many termite species construct well-designed nests. Especially, higher termites that belong to the family Termitidae, make various types of nests such as large mound nests or arboreal carton nests. There are only one species of termite that construct arboreal nests, i.e. Nasutitermes takasagoensis. This species belongs to the subfamily Nasutitermitinae, same as Hospitalitermes.

Black marching termites in Borneo

When I was a master-course graduate student, I was working in a Bornean rain forest, called “Bukit-Soeharto Protection Forest”, in East Kalimantan, Indonesia. The forest belongs to University Murawarman, which is a local university in East Kalimantan. There, I was studying on the foraging behavior of an interesting termite species Hospitalitermes medioflavus.

Before entering the graduate course, I had wanted to stydy in tropical rain forests, because I imagined that such rain forests showed that the highest diversity of organisms all over the world. So, I determined to join a lab at Univ Tokyo, which was studying on the ecology of social insects in tropics.

At first, I wanted to work on relatively large insects with interesting morphologies, like horned beetles, praying mantids, or leaf insects. However, it was very hard to find such insects. Instead, social insects like ants and termites were everywhere on the forest floor. But I didn’t want to work on such tiny bugs…

The only social insect species that attracted me is a black termteis that performed the large foraging march. They feed on lichens on tree trunks or rocks, and carry lichen foods in the form of small balls, back to their nests.

Hospitalitemes medioflavus

Thus, I decided to work on this termite species, firstly on the division of labor among workers when foraging. Among foraging workers, I found three types of workers, i.e. major, medium, and minor workers. Major and medium workers are engaged in a task carrying foodballs, while medium and small ones in a task gathering foods.

foraging castes of H. medioflavus

The above photo shows the foraging castes, including three types of workers and “nasute” soldiers with a frontal projection, from which defensive substances are discharged to repell predators.

My subsequent works revealed that medium and major workers are female, and minor workers and soldiers are male. Female major and medium workers, and male minor workers and soldiers are in the different stage of postembryonic development respectively.

In conclusion, it was shown that the eraborate social behavior was accomplished by the caste systems, which were determined by the sexes and the developmental stages of individual insects. Thus, my interest moved towards the developmental control of caste differentiation…

Giant mealworm

Zophobas morio

This insect is called “giant mealworm”, which is one of major food for reptiles, amphibians and fishes. It belongs to the order Coleoptera, the photo shows its larval form (final instar larva). The scientific name is Zophobas morio. We are keeping these worms for our bearded dragons. Everyday, dragons feed on them.

This is a very convenient species of insects to keep. We just put them into wheat bran. However, they often escape from the rearing boxes, so we often find them on the floor in my room and sometimes even on the passage in front of my room. In a extreme case, in the morning on one day, I found a worm on my BED!!!!! Probably, it entered my bag and I carried it to my home. Jesus!

I will later show the adult form some day…

pea aphid

Acyrthosiphon pisum

Aphids, well-known agricultural pests in temperate regions. They show fascinating and complicated life cycles, in which various phenotypes (or morphs) can be seen. For example, They overwinter as eggs on trees (primary hosts). When spring comes, hatched insects, that is called ‘founderess’ produce winged progenies via parthenogenetic viviparous reproduction.

Then, the winged ones disperse to secondary host plants (mainly grasses), where they produce lots of offsprings in the same way (viviparous parthenogenesis), over generations. During these stages in summer, they can change their phenotypes depending on the density conditions. If the population density is high, winged insects appear. While, the low density induces wingless insects.

When fall comes, the short-day condition induces the special females, that again disperse to the primary host plants and parthenogenetically produce both male and female individuals. Then, they mate and produce overwintering eggs.

aphid life cycle

In our lab, we are working on the developmental mechanisms producing various phenotypes. We are using the pea aphid Acyrthosiphon pisum and the vetch aphid Megoura classicauda as material insects.

Several years ago, we compared the embryonic development in sexual eggs with that of parthenogenesis. See the following paper for details.

Miura T, Braendle C, Shingleton AW, Sisk G, Kambhampati S, Stern DL (2003) A comparison of parthenogenetic and sexual embryogenesis of the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidoidea). Journal of Experimental Zoology (Molecularand Developmental Evolution)295B: 59-81.

Japanese damp-wood termite

Hodotermopsis sjostedti

This is one of the focal termite species in my lab. We annually collect this species in Yaku-shima Island, which is located at southern part of Japan Archpelago.

There are various types of individuals even in a single colony, so-called “castes”. Main castes are king and queen (reproductive castes), soldiers and workers (sterile castes). Immature individuals are named “larvae” in termites, while immatures with wing buds/pads are termed “nymphs”.

In this species, workers are normally 7th instar larvae, that are still totipotent to differentiate into soldiers and reproductives. Therefore, this caste is called “pseudergate” that means “false workers”, because workers are normally sterile in other termite species.

Only in termties, interestingly, special developmental events can be observed during their postembryonic stages. That is, “stationary molts” and “regressive molts”. In stationary molts, for example, 7th-instar pseudergates molt into further pseudergate instar (8th instar), with little growth. In regressive molts, nymphs molt back to pseudergate stage, when they cease to differentiate into alates.

Thus, termites show very interesting developmental systems in their elaborate social systems and complicated life cycles.