@ARTICLE{TreeBASE2Ref22359, author = {Juan Carlos Villarreal and Susanne S Renner}, title = {Correlates of monoicy and dioicy in hornworts, the apparent sister group to vascular plants}, year = {2013}, keywords = {Chromosome counts, sexual systems, spore size, trait correlation}, doi = {}, url = {http://}, pmid = {}, journal = {BMC Evolutionary Biology}, volume = {}, number = {}, pages = {}, abstract = {Background: The distribution of gamete-producing organs on one or more individuals shapes breeding systems and hence patterns in genetic diversity among and within populations. Reconstructing the evolution of sexual systems is essential to identify ecological, macro and micro-evolutionary correlations in the transition between sexual systems. Among haploid dominant plants (?bryophytes?: liverworts, mosses and hornworts) separate sexes (dioicy) have evolved several times independently accounting for 68% of liverwort species, 57% of moss species, and 40% of hornwort species. The transitions in sexual systems and correlates with life-history traits have been addressed in liverworts and mosses. Additionally, the traditional view on ?bryophytes? that dioicy is ancestral and monoicy is a derived sexual system has been recently rebuked [1]. Here we use a phylogeny for 98 of the 200 species of hornworts, the sister group to vascular plants, representing roughly equal proportions of all monoicous and dioicous species, to test whether transition in sexual systems are uni-directional or highly labile and discuss the potential evolutionary implications of these findings. . We further test correlations between small spores, low diversification rates, and dioicy found in hornworts. Results: Numerous transitions from and to dioicy are found in hornworts. The transition rate from dioicy to monoicy in hornworts exceeds that in the opposite direction, while diversification rates did not differ with sexual system. Another correlation important in mosses, that between monoicy and polyploidy, apparently plays a small role in hornworts; of 20 species with chromosome counts, only one is polyploid, the monoicous Anthoceros punctatus. A trait correlation analysis on 5000 Bayesian trees weakly supported a correlation with spore size, as did simulations; a contingency test then revealed that, while sexual system depends on spore size, the opposite is not the case. Selection for numerous spores in dioicous species may explain this finding, although we found no evidence for increased antheridium-per-chamber numbers in dioicous species. Conclusions: Sexual systems in hornworts are labile, and suggestions from the 1980s of dioicy being ancestral in ?bryophytes? and of monoicy being the derived sexual system are no longer tenable. Instead, sexual systems in the three lineages of haploid-dominant land plants (liverworts, mosses, and hornworts) appear plastic with no single preferential evolutionarily direction } }

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