Yoshitaka Tsubaki

Decreased immune response as a proximate cost of copulation and oviposition in a damselfly

Abstract.Males and females of the Japanese calopterygid damselfly, Matrona basilaris japonica Fester, showed a rapid (within 24 h), and significant reduction in immune system function (encapsulation response) after reproductive activity (copulation or oviposition).

Reproductive interference determines persistence and exclusion in species interactions

1. Reproductive interference is a negative interspecific sexual interaction that adversely affects the fitness of males and females during reproductive process. Theoretical studies suggest that because reproductive interference is characterized by positive frequency dependence it is far more likely to cause species exclusion than the density dependence of resource competition. However, the respective contributions of resource competition and reproductive interference to species exclusion, which have been frequently observed in many competition studies, remain unclear. 2. We show that reproductive interference is a far more critical cause of species exclusion than resource competition in the competition between Callosobruchus bean weevil species. In competition experiments over several generations, we manipulated the initial relative abundance of the adzuki bean beetle, Callosobruchus chinensis, and the southern cowpea beetle, Callosobruchus maculatus. When the initial adult ratio of C. chinensis : C. maculatus were 6 : 2 and 4 : 4, C. chinensis excluded C. maculatus. However, when C. maculatus was four times more abundant than C. chinensis at the start, we observed the opposite outcome. 3. A behavioural experiment using adults of the two species revealed asymmetric reproductive interference. The fecundity and longevity of C. maculatus females, but not those of C. chinensis females, decreased when the females were kept with heterospecific males. Fecundities of females of both species decreased as the number of heterospecific males increased. In contrast, resource competition at the larval stage resulted in higher survival of C. maculatus than of C. chinensis. 4. These results suggest that the positive frequency-dependent effect of reproductive interference resulted in species exclusion, depending on the initial population ratio of the two species, and the asymmetry of the interference resulted in C. chinensis being dominant in this study, as in previous studies. Classical competition studies should be reviewed in light of this evidence for reproductive interference.

Investment in immune function under chronic and acute immune challenge in an insect

In this paper we investigate the relationship between the chronic burden of mid‐gut parasites (eugregarine trophozooites) and the effect of an acute haemolymph challenge (a nylon insert) on two important insect immune effector systems (phenol oxidase (PO) and the encapsulation response) in a field‐population of damselflies. PO levels in the haemolymph, and the magnitude of the encapsulation response were maintained, regardless of chronic and subsequent acute experimental immune challenges. The maintenance of these effector systems is therefore probably an important life‐history requirement in these damselflies. Investment in mid‐gut PO levels was significantly negatively related to the animals chronic parasite burden after an acute experimental challenge in the haemolymph, suggesting that maintaining PO levels across two physiological compartments (haemolymph and mid‐gut) is costly. The results suggest that the immune effector system activity in different physiological compartments in an insects body is affected by chronic parasite burdens in the face of the demands imposed by an acute immune insult.

Expression of a costly, plastic secondary sexual trait is correlated with age and condition in a damselfly with two male morphs

Males of the damselfly Mnais costalis Selys (Odonata: Calopterygidae) are morphologically and behaviourally polymorphic, typically existing as clear‐winged non‐territorial ‘sneaks’ and orange‐winged territorial ‘fighters’. The amount of orange pigment in the wing, as measured with a chromameter, varied between individuals, and decreased as the reproductive season progressed. Young individuals maintained in the laboratory on high or low nutrient diets differed in the amount of pigment that developed in the wing. Males in the high nutrient group developed darker wings faster than those in the low nutrient group. Young adults of both sexes and morphs were fed 14C‐radiolabelled tryptophan or tyrosine (precursors of the pigments ommochrome and melanin, respectively). Ommochrome was restricted to the pseudopterostigma of the males of both morphs and was not present in females. The presence of tyrosine in the wing cells of orange males, but not of clear males, indicated that the orange pigment is at least partly constituted from melanin. These data show that at least some pigment levels must be maintained continuously in the wings of orange males, and that maintenance is costly as it is compromised at low nutrient levels.

Effects of age and body size on the male territorial system of the dragonfly, Nannophya pygmaea rambur (Odonata: Libellulidae)

Abstract Nannophya pygmaea males defended a small area including small bodies of water. Females visited these sites to oviposit and were usually captured by territorial males. There was considerable turnover in territory ownership at some sites during the course of the reproductive season. Some changes in ownership occurred because an intruder defeated the resident in an aerial contest. Old males were often replaced by young ones. High-quality territories, which attracted more females than others, were highly contended and larger males usually occupied these territories. Larger males occupied high-quality territories for more days than smaller males. Smaller or old males adjusted their behaviour to their social environment, occupying low-quality territories or sneaking into established territories if excluded from favoured ones. As a result, larger males had a higher lifetime mating success than smaller ones. However, multiple regression analysis showed that most of the variation in male mating success could be explained by two variables related to weather conditions (number of synny days during the reproductive span of each male, and number of synny days while each male occupied a high-quality territory) and not by male size.

Longevity of Sperm Within the Female of the Melon Fly, Dacus cucurbitae (Diptera: Tephritidae), and its Relevance to Sperm Competition

Sperm competition is the competition between sperm from the ejaculates of different males over fertilization of a limited number of ova (Parker, 1970). The probability of paternity of a second male after the female has been mated previously to a first male are often referred as the P2 value (Boorman and Parker, 1976) and has been measured in a wide range of animals (e.g., volume edited by Smith, 1984). The P2 value may vary depending on the mechanism of sperm transfer and on the structure of the male and female reproductive organs (Walker, 1980). There exists a considerable amount of data to show that the paternity of two males is determined in relation to mating order, and the last male advantage is often reported in many insect species (reviewed by Parker, 1970; Gwynne, 1984; Tsubaki, 1988). The mechanisms for this phenomenon are not yet clear except in some restricted cases. There is good evidence for mechanical displacement (removal or repositioning) of sperm of earlier males in many odonates (Waage 1979, 1984; Siva-Jothy, 1988: Siva-Jothy and Tsubaki. 1989) and at least one cricket (Ono et al.. 1989). On the other hand, some studies suggest that the amount of ejaculate is more important than mating order (Dickinson, 1986; Simmons, 1987; Tsubaki and Sokei. 1988). Although sperm viability and

Differences in adult and reproductive lifespan in the two male forms ofMnais pruinosa costalis selys (Odonata: Calopterygidae)

The damselflyMnais pruinosa costalis (Odonata: Calopterygidae) is unusual in that males are dimorphic, existing as clear-winged non-territorial ‘sneaks’ and orange-winged territorial ‘fighters’. Here we report the results of population census data and behavioural observations in the field and laboratory, and present estimates of emergence period, reproductive period, total lifespan, and reproductive success of each morph. Clear-winged males are smaller and have lower daily reproductive success than orange-winged males, but live for longer in the field and laboratory. We accounted for the difference in the ‘operational reproductive life’ of the two morphs and estimated lifetime reproductive success: there was no difference between clear-winged and orange-winged males. We discuss possible mechanisms for the maintenance of the two forms.

Group feeding as a strategy for exploiting food resources in the burnet moth Pryeria sinica

SummaryPryeria sinica (Zygaenidae) larvae feed on young and growing leaves of Euonymus japonicus in groups. The larvae often defoliate their host plant. Hence, the larvae are occasionally subject to serious shortage in food resources. We hypothesize that larval aggregation is an adaptation for the economical utilization of limited food resources. To test this hypothesis, the patterns of resource utilization were studied on larvae settled on shoots of host plant in various group sizes. The amount of food resources in a shoot was affected by the following three factors; (1) shoot growth, (2) food consumption of larvae, and (3) inhibition of growth or degeneration of shoot by larval activity on it. These factors were measured and the efficiency in resource utilization was compared among groups of varied sizes. The loss in resources caused by the third factor was found to be decreased with group size, i.e., the larger the group size, the larvae utilized the food resources more economically. The advantages of group feeding in survival and reproduction of this species were discussed using a simple graphical model.

Sperm competition in the melon fly, Bactrocera cucurbitae (Diptera : Tephritidae) : effects of sperm longevity' on sperm precedence

Sperm competition inBactrocera cucurbitae was studied by double matings of one female with normal and sterile males, with different intervals between the first and the second matings and with or without allowing oviposition after the first or the second mating. When the interval was less than 4 days, the last-male sperm precedence,P2, was not different from 0.5, but as the interval was prolonged,P2 was higher than 0.5. There was no significant difference between treatments in which females were allowed to oviposit after the first mating and only after the second mating. The reason for the higherP2 when the interval was long was therefore attributed not to sperm usage for egg fertilization during the two matings but, possibly, to sperm mortality. ThatP2 was 0.5 for shorter intervals suggests that particular sperm replacement mechanisms such as removal and inactivation are absent in B. cucurbitae. Our study is the first to demonstrate a significant effect of short sperm longevity on the last-male sperm precedence.

Some beneficial effects of aggregation in young larvae ofPryeria sinica moore (Lepidoptera: Zygaenidae)

The effects of group size on the survival and development of young larvae ofPryeria sinica Moore were investigated by laboratory and field experiments. Under laboratory conditions, about 20% of isolated larvae died of unsuccessful feeding in the first instar, however, larvae survived successfully in aggregations of four or more individuals. In the field, larvae emerge in early spring and wait for new leaves to open before feeding. In this period, the larger the group size of hatchlings the survival rate became higher. The nest-web spun by hatchlings was considered to play an important role in protecting them from desiccation. In the period that larvae began to feed on leaves, more than 36 larvae are necessary to aggregate for the successful establishment of feeding groups. The nest-web played an important role also in the establishment of feeding group. However, the natural group size of the first instar larvae was larger than the minimum group size to spin a sound nest-web in the field experiment. On the other hand, in later stage, larvae in a large group did not have an excess advantage in survival or developmental rate over larvae in a small group. It was found that the experiments on survival and developmental rates could not explain the reason that this species maintain large compact groups in the most part of larval period.