Epi-endophytic symbiosis between Laminariocolax aecidioides (Ectocarpales, Phaeophyceae) and Undaria pinnatifida (Laminariales, Phaeophyceae) growing on Argentinian coasts

The present is the first study on epi-endophytic algae on thalli of Undaria pinnatifida growing along Argentinian coasts. The main goal is to describe the nature and the morphology of this symbiosis. Individuals of Laminariocolax aecidioides were detected in both June and December 2004, growing on U. pinnatifida sporophytes. In nature, the epi-endophyte were macroscopically observed as dark zones that partially covered the hosts' fronds. L. aecidioides vegetative thalli were irregularly branched uniseriate filaments. The life cycle is described from laboratory cultures started from Patagonian populations. Caryology revealed that the sporophytic diploid phase presented 16 chromosomes whereas the gametophytic haploid phase presented 8 chromosomes. Isolates made from thalli growing in the interior of infected hosts developed into filamentous, branched sporophytes that reproduced by both unispores and plurispores that were produced in unilocular and plurilocular sporangia, respectively. The results of this paper also allowed us to conclude that L. aecidioides uses the thalli of U. pinnatifida as a proper substrate. The penetration of endophitic filaments among the hostCs cortical cells produced a lateral compression and, finally, their thalli development generated perforations in the host tissues. The effects of the epi-endophytic infection of L. aecidioides on U. pinnatifida are neither severe nor deleterious.