Poikilohydry in mosses: an ecological limitation or opportunity?

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Poikilohydry in mosses: an ecological limitation or opportunity?

Introduction

The most significant evolutionary event in the history of the plant kingdom began with the migration of aquatic photosynthetic organisms on to land approximately 475 Myr ago 1,2. Terrestrial embryophytes (referred to as embryophytes) derived from algal ancestors constitute two divisions, the bryophytes (lacking vascular tissue) and the tracheophytes (possessing vascular conducting tissues) 2. The evolutionary transition from aquatic photoautotrophs heralded the innovation of morphological and physiological characteristics critical for success in the terrestrial habitat, and consequent diversification of land plants. It has been hypothesised that early bryophyte related groups pioneered the transition from water to life on land 1,2,4. The mosses appear to be among the most ancient divisions of the land plants. Fossils of spores and gametophytes dating from the Mid Ordivician (~476 Myr) and Mid Sliurian (~432 Myr) have been discovered that closely resemble some modern bryophytes 4. Furthermore, the remains of fossilised spore-bearing organs and vascular tissues from the Lower Devonian (~410 Myr) reveal similarity in anatomical details to living mosses 4. This paleobotanical evidence combined with phylogenetic studies suggests that groups related to extant mosses are primitive ancestors of the major taxonomic groups of terrestrial plants.

In the comparatively uniform environment of aquatic life there is much less scope for variation in abiotic factors and hence natural selection. Light availability vertically through the water column is the major limiting factor. Photosynthetic organisms surrounded by water have no requirement for specialised anatomical features for water absorption, as diffusion occurs across the whole cellular surface, nor do aquatics require adaptive measures to protect against water loss through evaporation. Furthermore, a submersed plant requires no mechanical tissues as the buoyant force of water provides the necessary support 1,5. In contrast, plants entering the terrestrial environment are exposed to a greater variation in the fundamental abiotic conditions including moisture, temperature (rainforest and tundra), light and gravitation. In particular, the land environment exhibits dehydrating affects from the atmosphere and water may be continually lost through evaporation. Water and nutrients may be continuous underground, however, dependence on light requires the aboveground presence of photosynthetic tissue where water is often irregular 1,5. Therefore, suitable water relations are a prerequisite to survival on land. Embryophytes are able to tolerate the desiccating affects of the land environment and exhibit numerous morphological and physiological departures from the ancestral aquatic state. Depending on their ability to compensate for fluctuations in water supply and rate of evaporation, embryophytes may be classified as poikilohydric or homoiohydric 5.

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Poikilohydry as a limitation

Mosses represent the oldest living remnants of initial land colonisation. It can be viewed the mosses represent a primitive condition of plant development 4. Mosses can be said to represent an evolutionary intermediate between algal ancestors and the vascular plants, manifested in features that may be more in common with aquatic algae or lichens and are said not to be fully ‘terrestrialised’ or having a greater reliance on water 4,5,7,8. However, moss gametophytes do exhibit innovations such as tissue differentiation (leaves, rhizoids) and primitive vascularisation 3.

It is in the gametophyte form of mosses where ...

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