Abstract
Because stomata in bryophytes are uniquely located on sporangia, the physiological and evolutionary constraints placed on bryophyte stomata are fundamentally different from those on leaves of tracheophytes. Although losses of stomata have been documented in mosses, the extent to which this evolutionary process occurred remains relatively unexplored. We initiated this study by plotting the known occurrences of stomata loss and numbers per capsule on the most recent moss phylogeny. From this, we identified 40 families and 74 genera that lack stomata, of which at least 63 are independent losses. No trends in stomata losses or numbers are evident in any direction across moss diversity. Extant taxa in early divergent moss lineages either lack stomata or produce pseudostomata that do not form pores. The earliest land plant macrofossils from 400 ma exhibit similar sporangial morphologies and stomatal distribution to extant mosses, suggesting that the earliest mosses may have possessed and lost stomata as is common in the group. To understand why stomata are expendable in mosses, we conducted comparative anatomical studies on a range of mosses with and without stomata. We compared the anatomy of stomate and astomate taxa and the development of intercellular spaces, including substomatal cavities, across mosses. Two types of intercellular spaces that develop differently are seen in peristomate mosses, those associated with stomata and those that surround the spore sac. Capsule architecture in astomate mosses ranges from solid in the taxa in early divergent lineages to containing an internal space that is directly connected to the conducing tissue and is involved in capsule expansion and the nourishment, hydration and development of spores. This anatomy reveals there are different architectural arrangements of tissues within moss capsules that are equally effective in accomplishing the essential processes of sporogenesis and spore dispersal. Stomata are not foundational to these processes.
Highlights
Stomata in bryophytes are located on sporangia and are restricted in their occurrence across phylogeny
Species examined include the following, with the seven taxa lacking stomata denoted by asterisks: Takakia ceratophylla∗, Andreaea rothii∗, Sphagnum angustifolium∗, Polytrichastrum ohiensis, Atrichum angustatum∗, Tetraphis pellucida∗, Diphysium foliosum, Buxbaumia viridis, Physcomitrium pomiform, Physcomitrium (Physcomitrella) patens, Funaria hygrometrica, Dicranum scoparium, Orthotrichum pusillum, Plagiomnium cuspidatum, Ephemerum spinosum, Leucobryum glaucum∗, Bartramia pomiforme, Hypnum curvifolium, Brachythecium rutabulum, Thuidium delicatulum, and Neckeropsis undulata.∗ A KNOX mutant of P. patens that lacks stomata was acquired from Dr Neil Ashton
As the sister taxon to peristomate mosses, Oedipodium represents the earliest divergent moss lineage to possess stomata
Summary
Stomata in bryophytes are located on sporangia and are restricted in their occurrence across phylogeny. Liverworts are the only extant land plants that lack stomata entirely, while stomata are widespread but not ubiquitous in hornworts and mosses. With contemporary phylogenies pointing to hornworts as the earliest divergent bryophyte group (Puttick et al, 2018; Renzaglia et al, 2018), stomata are best interpreted as plesiomorphic in land plants, especially given that Leiosporoceros, the sister taxon to other hornworts, possesses stomata. Within the small hornwort clade of 10–12. Stomata Losses in Mosses genera there are two well-documented losses of stomata in derived taxa (Renzaglia et al, 2017). There are multiple moss orders and families that include taxa with and without stomata. Stomata are not vital to the survival and were not required for the initial radiation of bryophytes
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