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SummaryThe phytopathogenic fungus Ustilago maydis is obligately dependent on infection of maize to complete the sexual phase of its life cycle. Mating interactions between haploid, budding cells establish an infectious filamentous cell type that invades the host, induces large tumours and eventually forms large masses of black spores. The ability to switch from budding to filamentous growth is therefore critical for infection and completion of the life cycle, although the signals that influence the transition have not been identified from the host or the environment. We have found that growth in the presence of lipids promotes a filamentous phenotype that resembles the infectious cell type found in planta . In addition, the ability of the fungus to respond to lipids is dependent on both the cAMP signalling pathway and a Ras/MAPK pathway; these pathways are known to regulate mating, filamentous growth and pathogenesis in U. maydis . Overall, these results lead us to hypothesize that lipids may represent one of the signals that promote and maintain the filamentous growth of the fungus in the host environment.

The single-copy actin gene of Giardia lamblia lacks introns; it has an average of 58% amino acid identity with the actin of other species; and 49 of its amino acids can be aligned with the amino acids of a consensus sequence of heat shock protein 70. Analysis of the potential RNA secondary structure in the transcribed region of the G. lamblia actin gene and of the single-copy actin gene of nine other species did not reveal any conserved structures. The G. lamblia actin sequence was used to root the phylogenetic trees based on 65 actin protein sequences from 43 species. This tree is congruent with small-subunit rRNA trees in that it shows that oomycetes are not related to higher fungi; that kineto-platid protozoans, green plants, fungi and animals are monophyletic groups; and that the animal and fungal lineages share a more recent common ancestor than either does with the plant lineage. In contrast to small-subunit rRNA trees, this tree shows that slime molds diverged after the plant lineage. The slower rate of evolution of actin genes of slime molds relative to those of plants, fungi, and animals species might be responsible for this incongruent branching.

We review all instances in which the nuclear 5S rRNA genes of fungi, protist, nematode, and arthropod species have been reported to be linked to the tandemly repeated units of the rDNA, trans-spliced leader, and histone multigene families. The evolution of these gene arrangements is analyzed by mapping them to independently derived phylogenies. These analyses show that 5S rRNA genes have repeatedly become linked to diverse tandemly repeated gene families and that such linkages have also been subsequently inverted or lost in some species. These variable gene linkages are probably the result of stochastic gains and losses of variant repeat units, where functional 5S rRNA had transposed, by the mechanisms which are responsible for the concerted evolution of tandemly repeated multigene families. We discuss the possible mechanisms of 5S rRNA gene transposition and suggest that the characteristics of their promoter elements, transcription, and termination signals may allow functional copies of these genes to be fortuitously transposed through an RNA intermediate. We also review the evidence which shows that the linked 5S rRNA gene copies are transcribed. We conclude that the observed patterns of 5S rRNA gene linkages to the repeat units of other tandemly repeated multigene families have likely arisen due to fortuitous recombination events and are unlikely to represent the remnants of an eubacterial-like arrangement of rDNA operons or to have been established due to selective pressures.

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