Hemocyte load and immune resistance to Asobara tabida are correlated in species of the Drosophila melanogaster subgroup

Abstract

Larvae from six Drosophila species of the melanogaster subgroup were compared for both the hemolymph concentration of hemocytes and the ability to encapsulate the eggs of the parasitoid Asobara tabida (Hymenoptera; Braconidae). Results showed a high correlation between the parasitized hosts' concentration of circulating hemocytes and their aptitude to form a hemocytic capsule around the parasitic eggs. Two conditions seem to be required for the encapsulation of A. tabida eggs to succeed: one condition, which may relate to the recognition of the parasite by the host defense system, is the occurrence of a primary hemocytic response, which gives rise to the amplification of the hemocyte population; the other condition is the presence in the parasitized hosts of a hemocyte load large enough for the cellular capsule to be completed before the parasitic egg becomes protected by embedment within the host tissues. Since the concentration in hemocytes of the parasitized hosts is partially related to the concentration in hemocytes before parasitization, Drosophila species carrying a high hemocyte load could be better predisposed to resist A. tabida. Results are discussed in regard to the importance of a non-specific, quantitative character, such as the host hemocyte load, for the co-evolutionary immune interactions between A. tabida and its Drosophila hosts.

Introduction

The immune and physiological defense mechanisms represent the ultimate barrier to the development of endophagous parasites. In the immune defense of insects to endoparasitoids, both humoral and hemocytic components contribute to the recognition of the foreign invader and to its encapsulation (Brehélin et al., 1975; Brehélin, 1986; Ratcliffe and Götz, 1990; Russo et al., 1996; Strand and Pech, 1995), but the regulative factors and the underlying molecular mechanisms of the encapsulation reaction often remain undefined.

The hemocytes form the cellular component of the capsules surrounding parasitoids (Ratcliffe, 1993). They may also produce substances that favor encapsulation, such as the `encapsulation promoting factor' (EPF) found in hemocyte extracts (Davies et al., 1988; Strand and Pech, 1995). Other factors associated with hemocytes' encapsulation activity are enzymes, such as acid phosphatase, lysozyme, catalase, peroxidase (Crossley, 1979; Dvornik, 1992; Lackie, 1988) and the FAD-glucose dehydrogenase (Cox-Foster and Stehr, 1994), which are found in different hemocyte types, or eicosanoids, and which also influence their activity (Gadelhak et al., 1995; Miller et al., 1996). Phenoloxidase, which plays a major role in the metabolic pathway of melanin (Nappi et al., 1992; Ratcliffe, 1991; Söderhäll, 1982; Söderhäll and Smith, 1986; Sugumaran and Kanost, 1993), has its precursor, the prophenoloxidase, which can be partially released by hemocytes (Durrant et al., 1993; Iwama and Ashida, 1986; Leonard et al., 1985; Rizki and Rizki, 1959, Rizki and Rizki, 1980; Schmit and Ratcliffe, 1977). Therefore, we may wonder about quantitative variations that affect the hemocyte population during the encapsulation response.

In the Drosophila –parasitoid interaction, parasitization usually provokes a net increase in the concentration of the circulating hemocytes (Carton and Kitano, 1979; Eslin and Prévost, 1996; Nappi, 1981; Nappi and Carton, 1986). The synthesis of molecules such as lysozyme and phenoloxidase, which could be released by the hemocytes, are activated after parasitic aggression (Nappi and Carton, 1986; Ratcliffe et al., 1984). Our primary interest was therefore, to determine if the variation in Drosophila encapsulation ability (Boulétreau, 1986; Carton and Boulétreau, 1985; Carton and Kitano, 1981; Carton and Nappi, 1991; Kraaijeveld, 1994; Mollema, 1988) could be correlated with the variation in quantitative hemocytic factor.

This question was triggered by a previous result, which showed that, in D. simulans larvae parasitized by A. tabida, the most reactive hosts encapsulated A. tabida eggs within a few hours post-parasitization, and carried more circulating hemocytes than the less reactive larvae. In addition, we observed that D. melanogaster larvae, which usually fail to encapsulate A. tabida eggs, also carry fewer hemocytes than the resistant host, D. simulans. Therefore, it was suggested that the immune resistance of D. simulans to A. tabida could be caused by, at least partially, a high hemocyte load (Eslin and Prévost, 1996).

This question is of particular interest because the A. tabida egg displays an unusual `behavior' in the host hemocoel: right after being laid, the egg starts attaching to the host's organs and becomes totally embedded in the host tissues (Eslin et al., 1996; Kraaijeveld and Van Alphen, 1994; Monconduit and Prévost, 1994). We suggested that A. tabida eggs become protected from encapsulation by embedment within the host tissues, and that only host larvae with a high hemocyte load would be able to complete the encapsulation before A. tabida protection is achieved. This hypothesis was tested by comparing both the hemocyte load and the ability to encapsulate A. tabida eggs between six Drosophila species.

The six tested Drosophila species belong to the melanogaster subgroup and are closely related. However, only two siblings, D. melanogaster and D. simulans, occur in a European habitat where A. tabida lives (Mollema, 1988). Comparison between sympatric vs. allopatric hosts of A. tabida suggested that host–parasitoid co-evolutionary interactions may have influenced the host immune response.