Climbing the walls : behavioural manipulation of insects by baculoviruses

Parasites often have severe effects on their hosts by causing developmental and physiological alterations in their hosts. These alterations may contribute to parasite growth, reproduction and survival. For example, host development may be inhibited so that more nutrients become available for the parasite. Parasites can also interfere with host behavior as a strategy to increase parasite survival or transmission. This phenomenon is known as ‘parasitic manipulation’ or ‘behavioural manipulation’. Although many examples of parasitic manipulation are known, the genetic basis underlying such manipulations is largely unexplored. A thorough understanding of how parasites manipulate their hosts’ behavior is therefore lacking, but it can be hypothesized that parasites carry specific genes that induce these behavioural alterations. Such ‘behavioural’ parasite genes likely affect one or more host proteins directly or via the expression of specific target genes in the host, which subsequently leads to altered behaviour. Understanding the details of such interactions between parasite and host is important as parasitic manipulation is thought to be wide spread in nature and to be a strong driver of the co-evolutionary arms race between parasite and host. Furthermore, the strategies employed by parasites to alter behavior likely provide important insights in the molecular mechanism of many behavioural processes. Chapter 2 of this thesis reviews our current understanding of the mechanisms of behavioural manipulation in invertebrates. It discusses known examples of behavioural manipulation and the present knowledge on the mechanistic basis of these manipulations. Furthermore, an overview of host genes and proteins that play a conserved role in behavioural traits in different invertebrate species is given. These genes and proteins are worthwhile to be studied in more detail in the context of parasitic manipulation, as they might be suitable targets for parasites to induce behavioural changes.

 

This thesis focuses on behavioural manipulation in insect hosts by baculoviruses. Baculoviruses are DNA viruses that infect the larval stages of mainly lepidopteran insects. These viruses alter host behaviour in multiple ways. They induce hyperactivity in the larvae, which likely contributes to virus dispersal over a large area. In addition, baculoviruses alter host climbing behaviour leading to death at elevated positions, a phenomenon known as ‘tree-top disease’ or ‘Wipfelkrankheit’. This latter manipulation is thought to contribute to optimal virus dispersal on plant foliage. In the research described in this thesis baculoviruses and their lepidopteran insect hosts are used as a model system to study molecular mechanisms of behavioural manipulation. In Chapter 3 of this thesis the involvement of the protein tyrosine phosphatase (ptp) gene from the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in the induction of hyperactivity of the beet armyworm Spodoptera exigua was studied. A homolog of this gene in Bombyx mori nucleopolyhedrovirus (BmNPV) was previously shown to be important in hyperactivity in the silkworm B. mori. The results in Chapter 3 showed that the AcMNPV ptp gene induces hyperactive behaviour in S. exigua larvae and that the phosphatase activity of the encoded PTP enzyme is crucial for this behavioural change. Phylogenetic inference revealed that the baculovirus ptp is presumably transferred from a (ancestral) lepidopteran host to a baculovirus. Within the family Baculoviridae, ptp is only present in group I NPVs, which are a group of phylogenetically related baculoviruses. It is hypothesized that ptp-induced hyperactivity is an evolutionarily conserved strategy of group I NPVs to manipulate host behaviour.

 

To obtain insights into the target proteins of the baculovirus PTP enzyme to achieve hyperactive behaviour in infected caterpillars, a PTP substrate analysis was performed. Chapter 4 describes host and viral proteins that were found to co-purify with AcMNPV PTP. Many of these host proteins are known to be important in signalling pathways and behavioural traits. For one of these proteins, 14-3-3 z, mRNA transcript levels were found to be significantly higher in wild type AcMNPV-infected S. exigua larvae as compared to larvae infected with a mutant virus from which the ptp gene has been deleted (AcMNPV Δptp). The 14-3-3 protein is a known activator of the enzymes tryptophan hydroxylase and tyrosine hydroxylase, which in turn are required for synthesis of the neurotransmitters serotonin and dopamine. These signalling molecules are both important determinants in hyperactive behaviour in various organisms, and are associated with behavioural manipulation in several parasite-host systems. In Chapter 9 a model is proposed of how the putative interaction between baculovirus PTP and host 14-3-3 zmay lead to hyperactive behaviour.

 

Within the baculoviruses two different genes that encode protein tyrosine phosphatases, ptp and ptp2, are found. While the ptp gene induces hyperactivity (described in Chapter 3), no function has yet been assigned to the ptp2 gene. Chapter 5 describes the functional

characterization of the baculovirus ptp2 gene. PTP2 protein carries a conserved consensus sequence that is characteristic for mitogen-activated protein kinase (MAPK) phosphatases. SeMNPV ptp2 induced a mild apoptosis and caspase activation in insect cells upon transient expression, which may be related to its putative function as MAPK phosphatase. Several host proteins that co-purified with SeMNPV PTP2 have known functions in apoptosis and/or MAPK signalling, rendering them promising candidate proteins to be involved in SeMNPV PTP2-induced apoptosis and possibly MAPK signalling. Whether PTP2 also has any behavioural effect is unknown, but the data from this chapter indicate that PTP2 likely has a cellular function during virus infection.

 

Baculoviruses are known to alter host climbing behaviour, commonly leading to death at elevated positions (tree-top disease). In Chapter 6 the hypothesis was tested that baculovirus-induced hyperactive behaviour and tree-top disease are induced by a single baculovirus gene. To this aim the effect of the hyperactivity-inducing ptp gene (Chapter 3) on tree-top disease was investigated. The results demonstrated that AcMNPV ptp, known to cause hyperactive behaviour in S. exigua, is not involved in tree-top disease in this host. This indicates that hyperactivity and tree-top disease induced by baculoviruses are governed by independent mechanisms. Furthermore, a moulting-dependent effect on tree-top disease in S. exigua was found, which may relate to physiological and/or ecological differences between moulted and unmoulted larvae. In the next chapter (Chapter 7) the effect of AcMNPV infection on tree-top disease was investigated for two different host species, Trichoplusia ni and S. exigua. Data show that in T. ni larvae AcMNPV induces tree-top disease, causing death at elevated positions. In contrast, in S. exigua a moulting-dependent effect on the height at death was observed, as was also described in Chapter 6. Furthermore, in this chapter the role of the AcMNPV egt gene, encoding ecdysteroid UDP glucosyl transferase, on tree-top disease in T. ni and S. exigua larvae was analysed. A homolog of this gene causes tree-top disease in Lymantria dispar larvae infected with L. dispar (Ld) MNPV. The results (Chapter 7) show that AcMNPV egt does not play a role in the observed death at elevated positions in the two host systems studied. This indicates that the role of egt in tree-top disease may not be conserved among members of the family Baculoviridae.

 

 

In addition to the mechanisms employed by the generalist baculovirus AcMNPV to alter climbing behaviour, the effect of the specialist baculovirus S. exigua (Se) MNPV on tree-top disease in its only known host S. exigua was studied. In Chapter 8 it is shown that SeMNPV induces tree-top disease by triggering an aberrant response to light, and this positive phototaxis leads to death at elevated positions. A hypothesis is put forward that SeMNPV hijacks a host behavioural pathway that is involved in light perception to induce this positive phototactic response.

 

Overall, the results of this thesis show that hyperactivity and tree-top disease are induced by baculoviruses through independent mechanisms and that distinct baculovirus species presumably use different genes and proximate mechanisms to induce tree-top disease. While the baculovirus ptp gene induces hyperactivity, possibly by targeting host 14-3-3 z, the baculovirus ptp2 gene may function as a pro-apoptotic gene. The baculovirus egt gene does not have a conserved function in tree-top disease, indicating that other viral genes may underlie this host manipulative strategy. This thesis also demonstrates that tree-top disease in SeMNPV-infected caterpillars is the result of a strong attraction to light.

 

Parasitic manipulation is a fascinating biological phenomenon that can provide crucial information on how behavioural traits are controlled at the molecular level. The research described in this thesis provides several new insights in the mechanisms by which parasites manipulate the behaviour of their hosts.