Bryological and Lichenological Section/ABLS
Villarreal, Juan Carlos , Renzaglia, Karen S. .
Structural innovations in the gametophyte of Leiosporoceros dussii (Steph.) Hässel: new insights into hornwort evolution.
Known primarily from features of the sporophyte, Leiosporoceros dussii is the only hornwort with smooth spores in bilateral-alternate opposite tetrads. Recent molecular analyses identify a wide genetic separation between this species and other hornworts; chloroplast sequence data resolve Leiosporoceros Hässel as sister to the remaining hornworts and reveal a lack of RNA editing that is widespread in other members of the group. This study presents new findings on morphological and ultrastructural innovations in the gametophyte that further underscore the phylogenetic significance of this isolated taxon. The chloroplasts lack pyrenoids and resemble those in the "Phaeomegaceros" clade. Grana are aggregated in the center of the organelle with starch organized around the periphery. Grana end membranes that are absent from other hornwort chloroplasts are visible in Leiosporoceros. Nostoc colonies are found in median schizogenous canals that run the length of the thallus and represent a novel arrangement not previously reported for any land plant. The cyanobacteria invade sporelings through stoma-like clefts near the growing tip, and the Nostoc strands elongate and branch in synchrony with apical growth. No clefts are visible in older plants. In contrast, all other hornworts have globose Nostoc colonies that independently invade the thallus via ventral stoma-like clefts. Multiple antheridia, with as many as 20-25 per chamber, are shared only with Anthoceros and Folioceros. The evolution of gametophytic features such as chloroplast structure, Nostoc-hornwort symbiosis, schizogenous canals and antheridial features will be evaluated in light of recent molecular phylogenies. Leiosporoceros exhibits a number of unique features (Nostoc strands, grana end membranes, absence of clefts in adult plants) that extend the realm of known structural innovations within hornworts.
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1 - Southern Illinois University, Department of Plant Biology, 1125 Lincoln Drive, Carbondale, Illinois, 62901-6509, USA
Presentation Type: Oral Paper
Location: Salon A - Gov Ballroom/Hilton
Date: Monday, August 15th, 2005
Time: 3:30 PM