This study evaluates the hypothesis that spore coalescence may cause intraclonal variation. Spore coalescence might allow the occurrence of unitary thalli that in fact correspond to genetically different, coalesced individuals. Plant portions simultaneously derived from these chimeric individuals may exhibit dissimilar growth responses even when incubated under similar abiotic conditions. Testing of the hypothesis included various approaches. Transmission electron microscopy observations of early stages of sporeling coalescence indicated that polysporic plantlets were formed by groups of spores and their derivatives. Even though adjacent cells in two different groups may fuse, these groups maintained an independent capacity to grow and form uprights. Laboratory‐grown plantlets showed a significant correlation between the initial number of spores and the total number of erect axes differentiated from the sporeling. Construction and growth of bicolor individuals indicated the chimeric nature of the coalesced individuals. Coalesced, bicolor holdfasts had green and red cells, which subsequently produced green and red uprights, respectively. Individuals fronds were also chimeric, as indicated by the production of green and red branchlets from single, red uprights. The existence of mixed tissues was further substantiated by random amplified polymorphic DNA analysis. The banding pattern produced by branchlets of a unisporic thallus was consistently monomorphic, whereas the patterns produced by the polysporic thallus were polymorphic. Growth rates of polysporic thalli had larger data dispersal and variation coefficients than oligosporic or monosporic thalli. Therefore, all results support the original hypothesis and suggest that coalescence might be ecologically more important than previously thought.
Cells of the parasitic, unicellular eukaryote Ichthyodinium chabelardi were isolated from eggs of sardine (Sardina pilchardus) and from a previously unrecognized host, bogue (Boops boops), off the Atlantic coast of Portugal. Immediately after release from the infected fish egg or newly hatched larva, I. chabelardi cells were spherical and non-motile. After few minutes, spherical cells became flagellated and motile. Following 2-3 days of incubation and several divisions, spherical flagellated cells developed a twisted elongate shape and moved vigorously. Sequences of the small-subunit ribosomal RNA gene (SSU rDNA) were identical for I. chabelardi of both hosts and so were sequences of ITS1, ITS2 and the 5.8S rRNA gene. This genetic similarity suggests that eggs of sardine and bogue were infected by one single population of I. chabelardi. The SSU rRNA gene sequence of I. chabelardi was, in turn, 97% similar to those of two identical Asian isolates of Ichthyodinium sp. Phylogenetic analyses showed high support for the inclusion of Ichthyodinium in the so-called Marine Alveolate Group I (MAGI). Two morphologically well-described genera, namely Ichthyodinium and Dubosquella, have now been shown to belong to this group of seemingly exclusively parasitic alveolates.
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