Loculoascomycete origins and evolution of filamentous ascomycete morphology based on 18S rRNA gene sequence data

The fungal subclass Loculoascomycetes is characterized by asci or sexual spore sacks with two separable wall layers. At maturity, the inner wall layer protrudes out beyond the outer wall as in a jack-in-the-box. If the Loculoascomycetes were monophyletic and their jack-in-the-box type asci evolved once, then taxa from diverse loculoascomycete lineages would cluster together in a DNA sequence-based tree. To evaluate the phylogenetic history of the two-walled asci, I sequenced the 18S nuclear rRNA genes of 16 species from seven families in the loculoascomycete orders Pleosporales, Dothideales, and Chaetothyriales. Within the Loculoascomycetes, the Pleosporales form a monophyletic group in 99% of the bootstrapped parsimony trees. The Dothideales usually appear as a monophyletic group but without statistical support. Capronia pilosella (Herpotrichiellaceae, Order Chaetothyriales) clusters with plectomycete members of the subclass Euascomycetes rather than the other Loculoascomycetes in 99% of parsimony and neighbor-joining bootstrap replicates. Although the jack-in-the-box-type ascus is a good marker for large, monophyletic loculoascomycete orders, it must have evolved at least twice or been lost at least once.