SUMMARY: Free-swimming zoospores of several isolates of the oomycetous water mould demonstrate mutual attraction (adelphotaxis) which may result in auto-aggregation. In the apparent absence of exogenous chemotactic signals macroscopic globular aggregates or microscopic plaques of cysts are formed. Auto-aggregation was favoured by high concentrations of zoospores in suspension; the growth rate of spontaneous aggregates was estimated at 4--10 x 10spores min; growth of an aggregate was limited to about 7 min. By means of chemotactic assays, attractant activity for zoospores was detected in the vicinity of aggregates, and in zoospore or cyst supernatants. The attractant activity extracted from spore aggregates was heat-stable. In the absence of exogenous nutrients, the macroscopic aggregates of an homothallic isolate produced sporangia and oogonia. Aggregation was readily initiated as a result of zoospore chemotaxis towards capillary tubes containing exogenous attractant in an agar medium. Aggregating zoospores frequently assumed a slender swarm configuration moving directly towards the aggregate at the capillary-tube tip. Analysis of video recordings showed that many zoospores joining the swarm were attracted directly by the swarm rather than by the exogenous attractant. Such aggregation was a time-limited process, the duration of which was extended by the presence of amino acids in the zoospore suspension. Under natural conditions zoospore aggregation may be a dual-component chemotactic process triggered by exogenous attractants and amplified by chemical signals from the aggregating spores. The possible consequences of adelphotaxis and auto-aggregate formation, namely enhanced inoculum potential, syngamy and spore production, are discussed.


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