The hornworts: important advancements in early land plant evolution

In the last few years, a number of high-profile studies focused on land plant evolution have highlighted novel aspects of hornwort biology and the crucial position of this group among early land plants. For example, the widely accepted comprehensive molecular analysis by Qiu et al. (2006) which robustly recovered hornworts as sister to tracheophytes has recently been challenged. Using a phylotranscriptomic approach, Wickett et al. (2014) analyzed up to 852 nuclear genes from 92 taxa, and resolved hornworts as sister to land plants, sister to tracheophytes, or part of a bryophyte clade. Similarly, Cox et al. (2014) reanalyzed data published by Qiu et al. (2006) and others and recovered the monophyly of bryophytes. These most recent studies have rekindled the debate about bryophyte interrelationships, while other comprehensive studies on hornworts have advanced the molecular and evolutionary frontiers in unexpected ways. In this review, we discuss emerging lines of inquiry that place hornworts in the centre stage and concentrate on pervasive and long-standing evolutionary issues. The five aspects of the review are: 1) the contentious position of hornworts in the global phylogeny of land plants, 2) revolutionary findings on horizontal gene transfer, 3) evolution of the peculiar hornwort plastid, 4) symbiotic relationships with fungal and cyanobacterial endophytes, and 5) new insights on hornwort stomata and clefts in the gametophyte thallus.