Ascus and Ascospore Development of Five Species of the Lichen-Forming Genus Pertusaria

Abstract

The development of asci and spores was followed in Pertusaria velata (Turn.) Nyl., P. laevigata (Nyl.) Am., P. pertusa (L.) Tuck., P. pustulata (Ach.) Duby, and P. alpina Hepp. In the one- and two-spored species (P. velata, P. laevigata, P. pertusa, P. pustulata) spore formation began after division of the fusion nucleus. Each spore could enclose one to four nuclei initially, and as the spore matured the nuclei divided until there were 100-200 per spore. The eight- spored species (P. alpina) initially formed eight uninucleate spores. The nucleus of each spore divided until each had approximately 25 nuclei. There was no evidence that the spindle apparatus took part in spore formation of the one- and two-spored species. Three basic types of ascus initials were found; a proposed evolutionary sequence of these is given. The development of P. laevigata differed from that of all other species investigated by an apparent seasonal production of asci and apothecia. The study of the development of asci and apothecia in the lichen-forming fungi has been neglected in recent years. The modern work dating from 1940, and represent- ing the efforts of only seven authors, has followed a pattern similar to the earlier studies (of lichen-forming fungi) with the result that it has been impossible to assess the importance of the data in regard to evolutionary patterns within a family or genus of lichens. (For a review of the earlier studies see Erbisch (1966) and Smith (1921). This has been mainly due to the fact that in all the studies no extended investigation of a single genus was accomplished. The supposed evolutionary significance of the development of ascocarps and asci in the nonlichen-forming ascomycetes and the absence of such studies in the lichen- forming ascomycetes indicated that a study of ascus development of several species of one lichen genus could prove valuable. Therefore, a study was carried out upon five species of Pertusaria, a genus providing material not only for a study of the formation of the usual eight-spored ascus but also for variations upon this pattern.