Climate change effects on host parasite interactions (Alleen Engelse tekst)

An increasing number of studies indicate that climate change may strongly influence the condition and thus the ecosystem functioning of Dutch lakes. Since 1960, the average temperature of European lakes has risen more than one degree and the increase in winter and spring water temperature has been shown to be relevant. Elevated temperatures in that period of the year can lead to marked changes in the dynamics of phytoplankton, thus altering the timing and/or occurrence of distinctive successional events, which again will result in changing the outcome of species interactions such as parasitism.
Furthermore, changes in environmental conditions and in species interactions do not only show an impact on the population level, but are also important in shaping and maintaining genetic diversity of natural populations. Differential reproductive success of host and parasite clones may be dependent on their interaction with each other in form of GenotypeHost by GenotypeParasite interactions (GH x GP), but may also be dependent on their interaction with environmental factors as Genotype by Environment interactions (GH x GP x E).

The research is based on a host –parasite model system which has been monitored intermittently over the last 20 years in Lake Maarsseveen. The host is a colony forming freshwater diatom Asterionella formosa, which typically blooms twice a year, in early spring and again in early fall. These blooms are host to the chytrid parasite Zygorhizidium planktonicum, which is able to become epidemic at specific light and temperature conditions. The chytrid is a host specific, obligatory killing parasite which belongs to the primitive fungi. It can reproduce both sexually and asexually, in both cases producing a motile lifestage, zoospores, which can swim actively and find new hosts.

Picture 1: sporangia of Z. planktonicum in phase contrast (left) and in fluorescence microscopy

Figure 1: Life cycle of Zygorhizidium planktonicum (Doggett and Porter 1996)

Asexual reproductive cycle (A–D): A zoospore infects its host and forms uninucleate haploid thalli (A) which differentiate into a sporangium (B). By dehiscence (C), the sporangium releases again uniflagellate zoospores (D).
Sexual reproductive cycle (E-I): Two heterothallic gametangia (E) fuse by means of a single epibiotic conjugation
tube (F). After plasmogamy a thick walled resting spore is formed (G), the nuclei fuse and the zygote enters a stage of dormancy (H). Meiosis occurs within the newly formed meiosporangium after resting spore germination (I). The haploid zoospores are released by dehiscence (J).

During an epidemic, the parasite can infect close to 100% of the host population, and thus may act as a strong selective agent on the host population. This parasite mediated selection on the host genotypes may be confounded by the differential response of host genotypes to environmental factors such as temperature, light or nutrient availability.

Objectives
Within this project, we aim to disentangle environment mediated and parasite mediated selection of A. formosa genotypes on basis of laboratory GH x GP x E experiments in combination with monitoring the natural host-parasite dynamics in Lake Maarsseveen.

GHxGPxE interaction experiments:
In a first step we want to disentangle the influence of environmental factors such as temperature, light and phosphorus availability on growthrates of several A.formosa clones. In a second phase, we will then test for differential infectivity of Z. planktonicum clones on different A. formosa clones and how these respond to environmental gradients.

Picture 2: uninfected A. formosa culture (left) and the effect of fungal parasitism on a host culture of the same age (right)

Monitoring host-parasite dynamics in Lake Maarsseveen
The Maarsseveense Plas is a 32m deep lake which was created by sand excavation around the 1960's and is situated about 4 km NNE of Utrecht.
By monitoring the host parasite dynamics in the lake on a weekly basis, we hope to find out which biological and physical conditions regulate the host and parasite populations.

Employed techniques
Plankton culture and sampling, isolation of clones, microscopy, batch culturing, carbon/nitrogen analyser, silicate analysis, photospectrometer, PHYTO-PAM,data analysis, practical fieldwork on Lake Maarsseveen.

Duration
Internship projects can last from 2-6 months, work-plans and exact research questions will be discussed jointly. The location is Nieuwersluis.

Information
For further information you can contact Alena Gsell (PhD student), tel: +31 (0)294 239 393 or e-mail: a.gsell@nioo.knaw.nl