Ranking higher taxa using divergence times:a case study in Dothideomycetes

The current classification system for the recognition of taxonomic ranks among fungi,especially at highranking level,is subjective.With the development of molecular approaches and the availability of fossil calibration data,the use of divergence times as a universally standardized criterion for ranking taxa has now become possible.We can therefore date the origin of Ascomycota lineages by using molecular clock methods and establish the divergence times for the orders and families of Dothideomycetes.We chose Dothideomycetes,the largest class of the phylum Ascomycota,which contains 32 orders,to establish ages at which points orders have split;and Pleosporales,the largest order of Dothideomycetes with 55 families,to establish family divergence times.We have assembled a multi-gene data set(LSU,SSU,TEF1 and RPB2)from 391 taxa representing most family groups of Dothideomycetes and utilized fossil calibration points solely from within the ascomycetes and a Bayesian approach to establish divergence times of Dothideomycetes lineages.Two separated datasets were analysed:(i)272 taxa representing 32 orders of Dothideomycetes were included for the order level analysis,and(ii)191 taxa representing 55 families of Pleosporales were included for the family level analysis.Our results indicate that divergence times(crown age)for most orders(20 out of 32,or 63%)are between 100 and 220 Mya,while divergence times for most families(39 out of 55,or 71%)are between 20 and 100 Mya.We believe that divergence times can provide additional evidence to support establishment of higher level taxa,such as families,orders and classes.Taking advantage of this added approach,we can strive towards establishing a standardized taxonomic system both within and outside Fungi.In this study we found that molecular dating coupled with phylogenetic inferences provides no support for the taxonomic status of two currently recognized orders,namely Bezerromycetales and Wiesneriomycetales and these are treated as synonyms of Tubeufiales while Asterotexiales is treated as a synonym of Asterinales.In addition,we provide an updated phylogenetic assessment of Dothideomycetes previously published as the Families of Dothideomycetes in 2013 with a further ten orders and 35 families.