Proposed parallel evolution of Rhodophyta and Eumycota

There are a number of features both groups have in common, morphological as well as biochemical. Substances found in both as primary gene products or secondary metabolism were listed by Desmoulin in 2 publications (197., 19782). The morphological and karyological features include  incomplete septa in filamentous taxa, though the way to close the central pit during most of their lifetime is different: formation of pit plugs with a very distinctive structure and chemism in Rhodophyta, no such structures in Ascomycota, clamp connections in the majority of Basidiomycota. There are also unicellular members in both groups, which may represent a primitive state or be the result of reductive evotionary trends. During cell division the nuclear envelope remains intact, microtubular filaments involved in chromosome movements remain inside this envelope. There are no flagella in any stage of development, neither are centrioles present.  Morphology of the vegetative thallus follows the stages first descibed by Chadefaud from 1954 onwards, from unicellular through filamentous to cladomoid with a special structure called “incrustant” by him, that can be derived from a cladom. These terms never have been fully accepted by authors writing in English, though they are of primary importance.  In a cladom a central filamentous axis is developed, that is either composed of a single thread or by a bundle of parallel threads closely attached to each other. That axis usually is indeterminate in its length und producing 2 kinds of lateral structures, which may be either cylindrical or flattened, the first called pleuridia, the second phyllidia from their leaf-like shape. In Rhodophyta both types occur. They are distinguished from the axis by their strictly limited length. In Eumycota the filamentous type is prevailing in the vegetative part, pleuridia or phyllidia  are not known among them.  What Chadefaud calls “thème incrustant” is different from the cladom in that the central axis is flattend, composed of numerous threads (called hyphe in fungi) closely attached to each other, from which lateral branches with limited growth corresponding to pleuridia, which are arising from each more or less isodiametrical cell forming a surface layer. The flat axis usually is attached to the substrate, so that formation of the lateral branches is unilateral confined to the exposed surface. All these types are known from Rhodophyta. In fungi the crustose and a slight modification of the cladomoid type are confined to lichenized taxa. The modification results from an erect axis with short cells producing a closed layer of lateral branches in all directions, so that cylindrical stems and branches are formed like those of Cladonia.   The anatomical features of fruiting bodies especially in ascomycetes can easily be derived from these structures. In basidiomycetes crustose fruiting bodies are representing the incrusting type in almost its original manner. Stipitate fruiting bodies as found mainly in Agaricales are characterized by a different anatomy in the stipe, which is usually composed of parallel hyphae  , and the pileus with a dorsiventral differentiation in a cortex on one side, hyphae forming the hymenophore on the other. Closed perithecia, pseudothecia and pycnidia are composed of more or less intertwined hyphae as central axis of the encrusting type either forming an outward layer of short branches or being exposed on the outside (Lohwag’s derm and cutis respectively) and the hymenial parts in the interior. There are numerous modifications of that general scheme, that can’t be discussed in detail here. In Rhodophyta, conceptacles correspond to that general type, they come very close morphologically to perithecia with asci in genera like Hildenbrandia, though the organs produced within them have a very different function. Though the similarity is only one of gross morphology between reproductive organs in Rhodophyta and ascomycetes it is characteristic for a special stage of evolution, which is parallel in both major groups. The presence of plastids is unique to Rhodophyta, though and shared with Chlorophyta and Cyanophyta only.

It is hypothesized here that especially the absence of motile structures composed of microtubules are more primitive features compared to flagellate cells with flagella of the 9+2 strands type, which is different from functionally similar organelles in Prokaryota. The pigments in Rhodophyta are directly derived from those of Cyanobacteria. On this level of evolution horizontal gene transfer seems to have still been responsible for the acquisition of chemical and morphological features for both Rhodophyta and Fungi.


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