Myrionema strangulans (Chordariales, Phaeophyceae) epiphyte on Ulva spp. (Ulvophyceae) from Patagonian Atlantic coasts

Abstract

Fronds of Ulva lactuca L. from Patagonian Atlantic coasts exhibited brown spots produced by the presence of Myrionema strangulans Greville (Chordariales, Phaeophyceae. The occurrence of M. strangulans on U. lactuca is widely reported in several region of world but there were no detailed studies about the theme. In the present study we describe the morphology and interactions of M. strangulans with U. lactuca under light and electron microscopes and reconstructed in vitro in all stages of its life cycle. M. strangulans presented a prevalence of infection of 100%. In case of the strongest epiphytism, the host cuticle exhibited perforations, massive depigmentation, cellular disorganization and cuticle rupture. Under transmission electron microcopy, it was possible to demonstrate a purely epiphytic relationship between both organisms. M. strangulans formed discoid thalli constituted by vegetative filaments radiating from a central zone to a peripheral zone. Transversally, discs were formed by two strata: a basal monostromatic and a filamentous erect. From the monostromatic stratum, hyaline hairs and reproductive structures were generated. Both plurilocular and unilocular sporangia were observed. Zoids from plurilocular and unilocular sporangia were able to germinate in culture. Myrionema strangulans exhibited a haplo-diplontic and heteromorphyc life cicle, with thalli with three different morphologies. The haploid chromosome number was 12 ± 2 chromosomes.