ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01267.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01268.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01269.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe origin of terrestrial plants from a charophycean ancestor is assumed as a basis for the consideration of the origin of life histories amongst this group. Charophycean algae are vegetatively gametophytic, thus requiring the interpolation of the multicellular sporophytic stage. The model presented here derives the sporophyte and typical sporangial contents from the zygospore produced from the reproductive structure of a hypothetical charophycean land plant ancestor. The presence of monads, dyads, and various forms of tetrads in the fossil record of presumed land plants add support to this model. In addition, such spore types suggest that the temporal relationship of meiosis to sporopollenin deposition was less strictly controlled than in extant land plants.

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01270.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01271.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryLife histories of photosynthetic eukaryotes traditionally‐termed algae exhibit a considerably greater degree of complexity than those of ‘higher cryptogams.’ Some algae have a so‐called ‘obligate’alternation between spore‐producing and gamete‐producing phases, but the majority seem capable of following other pathways depending upon environmental conditions. In only four algal classes do life histories show a change in morphological and/or nuclear phases. The following basic life histories are recognized in the Chlorophyceae, Phaeophyceae and Rhodophyceae:(a) monophasic, a diploid or haploid phase, (b) two or more phases, most commonly an alternation of an isomorphic or heteromorphic haploid gametangial phase and a diploid sporangial phase, and (c) three phases (unique to florideophyte Rhodophyceae), with a diploid spore‐producing phase (carposporophyte) developing on the gametangial phase, a diploid phase (tetrasporophyte if meiosis is sporic) and a haploid gametangial phase. Evidence from recent research indicates that in many algae there is an uncoupling of the morphological and nuclear phases. The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis. Free‐living morphs in heteromorphic life histories may be morphologically so dissimilar that formerly they were attributed to different genera.Evolution of the carposporangial phase in red algae is speculated to be a means of achieving zygotic amplification to compensate for the infrequency of syngamy. Such amplification allows the production of a large number of dispersible products from a single fertilization. The direct development of a free‐living tetrasporangial phase is considered another mechanism for achieving amplification. In freshwater red algae the growth of an upright phase from a perennial microscopic one is considered an adaptation for maintaining their upstream position.Life history pathways in algae are controlled by subtle environmental influences (e.g. photoperiodism, temperature, light quality, nutrients). Experimental evidence is lacking to support the contention that spatial and/or temporal partitioning of the environment is a mechanism favouring the maintenance of heteromorphy. Herbivory is known to be an important selective force suppressing some morphs and accentuating the seasonal dominance of others. Differential resistance of morphs to herbivory in environments where grazing intensity is predictable may lead to the selective maintenance of heteromorphy.Algal life history patterns are unexplored in terms of evolutionary processes. Various models for the evolution of biphasic or polyphasic life histories stress the importance of the capacity for both asexual dispersal of successful genotypes and for the generation of new genotypes via meiosis and syngamy. All evidence points to the fact that many life history processes operative in algae differ significantly from those described

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01272.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryCurrent ideas on the evolution of alternation of generations in land plants are reviewed in the context of important recent advances in plant systematics and the discovery of remarkable new palaeobotanical evidence on early embryophyte life cycles. An overview of relationships in major groups of green plants is presented together with a brief review of the early fossil record as a prelude to discussing hypotheses of life cycle evolution. Recent discoveries of life cycles in the early fossil record are described and assessed. The newly discovered gametophyte and sporophyte associations are based on exceptionally well‐preserved material from the Rhynie Chert, Scotland (Middle Devonian: 380–408 Myr) and compression fossils from other Devonian localities. These data document diplobiontic life cycles in plants at the ‘protracheophyte’ and early tracheophyte level of organization. Furthermore, the early fossils have a more or less isomorphic alternation of generations, a striking departure from life cycles in extant embryophytes. This unexpected similarity between gametophyte and sporophyte calls for a cautious approach in identifying ploidy level in early groups. Viewed in a systematic context, the neontological and palaeontological data contribute towards the formulation of a coherent hypothesis of life cycle evolution in major, early embryophyte groups. Evidence from extant groups strongly supports a single direct origin of the diplobiontic life cycles of land plants from haploid, haplobiontic life cycles in ancestral ‘charophycean algae’. The interest of the new palaeobotanical data lies in its relevance to life cycle evolution at the restricted level of vascular plants rather than at the more general level of embryophytes (vascular plants plus ‘bryophytes’). The occurrence of morphologically complex, axial gametophytes in early vascular plants is consistent with the moss sister‐group proposed in some cladistic analyses. Similarities of moss gametophytes to fossils in the vascular plant stem‐group are discussed, and it is argued that the late appearance of mosses in the macrofossil record may be due to the problem of recognizing stem‐group taxa. The new palaeobotanical evidence conflicts with previous hypotheses based on extant groups that interpret morphological simplicity as the plesiomorphic condition in the gametophytes of vascular plants. These new data indicate that a significant elaboration of both gametophyte and sporophyte occurred early in the tracheophyte lineage, and that the gametophytes of extant ‘pteridophytes’ are highly reduced compared to those of some of the earliest ‘protracheophytes’. Vestiges of this early morphological complexity may remain in the gametophytes of some extant

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01273.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01274.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryArguments against the compiling of generalized life cycles summarizing alternation of generations in ferns are presented, and some common misconceptions about breeding systems addressed. What little is known or can be deduced about time frames, mechanisms and significance of alternation events in the lives of two species: bracken fern (Pteridium) and Killarney fern (Trichomanes speciosum) is presented. Evidence is provided that gametophytes may play a more important role in survival of both these species than previously suspected, and the need for more long‐term studies and experiments/measurements of ferns in natural conditions/populations is stresse

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01275.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Summary1In aggregate, past discussions of heterospory and its role in the alternation of generations are riddled with ambiguities that reflect overlap of terms and concepts. Heterosporysensu latocan be analyzed more effectively if it is fragmented into a series of more readily defined evolutionary innovations: heterosporysensu stricto(bimodality of spore size), dioicy, heterosporangy, endospory, monomegaspory, endomegasporangy, integumentation, lagenostomy,in situpollination,in situfertilization, pollen tube formation, and siphonogamy (Tables 1, 2, Figs 1, 13). Current evidence suggests that the last five characters are confined to the seed‐plants.2The fossil record documents repeated evolution of heterosporous lineages from anisomorphic homosporous ancestors. However, interpretation is hindered by disarticulation of fossil sporophytes, the difficulty of relating conspecific but physically independent sporophyte and gametophyte generations in free‐sporing pteridophytes, the inability to directly observe ontogeny, and the rarity of preservation of transient and/or microscopic reproductive phenomena such as syngamy and siphonogamy. Unfortunately, the rarely preserved phenomena are often of far greater biological significance than corresponding readily preserved phenomena (e.g. heterospory versus dioicy, heterosporangy versus endospory).3In most fossils gametophyte gender can only be inferred by extrapolation from the morphology of the sporophyte and especially of the spores. This is readily achieved for species possessing high‐level heterospory, when the two spore genders have diverged greatly in size, morphology, ultrastructure and developmental behaviour. However, the earliest stages in the evolution of heterospory, which are most likely to be elucidated in the early fossil record of land‐plants, also show least sporogenetic divergence. It is particularly difficult to distinguish large microspores and small megaspores from the large isospores of some contemporaneous homosporous species (Figs 3–6a,g). Heterospory is best identified in fossils by quantitative analysis of intrasporangial spore populations.4The spatial scale of the differential expression of megaspores and microspores varies from co‐occurrence in a single sporangium (anisospory) to different sporophytes (dioecy) (Figs 6–8). Studies of the relative positions of the two spore morphs on the sporophyte, and of developmentally anomalous terata (Fig. 9), demonstrate that gender is expressed epigenetically in both the sporophyte and gametophyte. Hormonal control operates via nutrient clines, with nutrient‐rich microenvironments favouring femaleness; megaspores and microspores compete for sporophytic resources. External environments can also influence gender, particularly in free‐living exosporic gametophytes.5The evolution of heterospory was highly iterative. The number of origins is best assessed via cladograms, but no current phylogeny includes sufficient relevant tracheophyte species. Also, several extant heterosporous species differ greatly from their closest relatives due to high degrees of ecological specialization and/or saltational evolution; extensive molecular data will be needed to ascertain their correct phylogenetic position. Current evidence suggests aminimumof 11 origins of heterospory, in the Zosterophyllopsida (1: Upper Devonian), Lycopsida (1: Upper Devonian), Sphenopsida (?2: Lower Carboniferous), Pteropsida (?4: Upper Cretaceous/Palaeogene) and Progymnospermopsida (?3: Upper Devonian/Carboniferous). The arguably monophyletic Gymnospermopsida probably inherited heterospory from their progymnospermopsid ancestor (Table 3, Figs 11–13). No origin of heterospory coincides with the origin of (and thus delimits) any taxonomic class of tracheophytes. The actual number of origins of heterospory is probably appreciably higher, exceeding that of any other key evolutionary innovation in land‐plants and offering an unusually good opportunity to infer evolutionary process from pattern.6Heterospory reflects the convergent attainment of similar modes of reproduction in phylogenetically disparate lineages. Nature repeated this experiment many times, whereas true phylogenetic synapomorphies evolve only once and require a unique causal explanation. Cladograms also offer the best means of determining the sequence of acquisition of heterosporic phenomena within lineages, here exemplified using the lycopsids (Fig. 10). Comparison of such sequencesamonglineages can potentially allow generalizations about underlying evolutionary mechanisms. Current evidence (albeit inadequate) indicates broadly similar sequences of character acquisitions in all lineages, though they differ in detail. Some logical evolutionarily stages were temporarily bypassed. Other lineages appear to have acquired two or more characters during a single saltational evolutionary event. Heterosporic phenomena can also be lost during evolution. Although no complete reversals to homospory have been documented, this could reflect unbreakable developmental canalization of heterospory rather than selective advantage relative to homospory. Several extant species refute widely held assumptions that certain phenomena, notably heterospory and dioicy, are reliably positively correlated. Moreover, some fossils are likely to possess combinations of heterosporic characters that are not found in their extant descendants. Fossil data have played a crucial role in understanding both the number of origins of heterospory and the ensuing patterns of character acquisition.7Although non‐adaptive evolutionary events are likely in at least some lineages, the highly iterative nature of heterospory and similar patterns of character acquisition in different lineages together suggest that its evolution was largely adaptively driven. However, many previous adaptive models of heterosporic evolution confused pattern and process, and paid insufficient attention to the role of the environment as a passive filter of novel morphotypes. Linear gradualistic models were imposed on the data, often intercalating hypothetical intermediates where desired.8The evolution of heterospory is best understood in terms of inherent antagonism between the sporophytic and gametophytic phases of the life history for control of sex ratio and reproductive timing. Control is achieved directly by the gametophyte, via gametogenesis, and indirectly by the sporophyte, via sporogenesis and the ability to determine to varying degrees the environment in which the gametophyte undergoes sexual reproduction. Increasing levels of heterospory (particularly the acquisition of endospory) compress the heteromorphic life history, as the increasingly dominant sporophyte progressively co‐opts the sex determination role of the gametophyte. The resulting life history is more holistic, effectively streamlining evolution by offering only a single target for selection.9However, by wresting control of sex ratios from the gametophyte, the ability of the sporophyte to respond rapidly to environmental changes decreases. This competitive weakness is greatest for heterosporous species possessing exosporic but obligately unisexual gametophytes (epitomized by the pteropsidPlatyzoma*). It can be alleviated in endosporic species by occupying favourable environments (e.g. the aquatic Salviniales and Marsileales), switching to an apomictic mode of reproduction (thereby incurring inbreeding depression; e.g. many selaginellaleans), or acquiring more complex pollination biologies (thereby by‐passing the environment as a selective filter: the seed‐plants).10Lineages differ greatly in the maximum number of heterosporic characters that were acquired by their most derived constituent species. Several Devono‐Carboniferous lineages reached the level of reducing numbers of functional megaspores to one per sporangium (Figs 7e,f, 8, 13), but only the putatively monophyletic gymnospermopsids broke through this apparent barrier to acquire the increasingly complex pollination biology that characterizes modern seed‐plants.11Many theories have been proposed to explain the remarkable success y and ecological dominance) of seed‐plants. The majority focus on characters that are absent from the earliest seed‐plants (the Devono‐Carboniferous lyginopterid pteridospermaleans), which were no more reproductively sophisticated than other penecontemporaneous lineages possessing advanced heterospory (particularly the most derived lycopsids, equisetaleans and progymnospermopsids). Reliable pollination was a key reproductive breakthrough, though the sophisticated economic‐vegetative characters inherited by the earliest seed‐plants from their putative progymnospermopsid ancestors were probably equally important in ensuring their success in water‐limited habitats.12With the exception of some ecologically specialized pteropsids, known heterosporous lineages originated during a relatively short period in the Upper Devonian and Carboniferous (Fig. 11). They exploited a window of opportunity that existed before niches became too finely partitioned and saturated with seed‐plant species. This non‐uniformitarian ecology renders negligible the probability of new heterosporous lineages becomingestablishedtoday, even though ‘hopeful monsters’ possessing ‘incipient heterospory’ are probably constantly being generated f

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1994.tb01276.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY