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Kazumi Matsuoka - One of the best experts on this subject based on the ideXlab platform.
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cyst theca relationship and phylogenetic position of impagidinium caspienense incubated from caspian sea surface sediments relation to gonyaulax baltica and evidence for Heterospory within gonyaulacoid dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
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Cyst‐Theca Relationship and Phylogenetic Position of Impagidinium caspienense Incubated from Caspian Sea Surface Sediments: Relation to Gonyaulax baltica and Evidence for Heterospory within Gonyaulacoid Dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
Anna J. Pieńkowski - One of the best experts on this subject based on the ideXlab platform.
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cyst theca relationship and phylogenetic position of impagidinium caspienense incubated from caspian sea surface sediments relation to gonyaulax baltica and evidence for Heterospory within gonyaulacoid dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
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Cyst‐Theca Relationship and Phylogenetic Position of Impagidinium caspienense Incubated from Caspian Sea Surface Sediments: Relation to Gonyaulax baltica and Evidence for Heterospory within Gonyaulacoid Dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
Kenneth Neil Mertens - One of the best experts on this subject based on the ideXlab platform.
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cyst theca relationship and phylogenetic position of impagidinium caspienense incubated from caspian sea surface sediments relation to gonyaulax baltica and evidence for Heterospory within gonyaulacoid dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
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Cyst‐Theca Relationship and Phylogenetic Position of Impagidinium caspienense Incubated from Caspian Sea Surface Sediments: Relation to Gonyaulax baltica and Evidence for Heterospory within Gonyaulacoid Dinoflagellates
Journal of Eukaryotic Microbiology, 2017Co-Authors: Kenneth Neil Mertens, Yoshihito Takano, Siamak Bagheri, Vera Pospelova, Anna J. Pieńkowski, Suzanne A.g. Leroy, Kazumi MatsuokaAbstract:We investigate the cyst-theca relationship of Impagidinium caspienense. Through an incubation experiment, we succeeded in examining the motile stage. Additional molecular analysis of single-cyst PCR (LSU and SSU rDNA) reveal that the cyst is related to the species Gonyaulax baltica Ellegaard et al. (2002). The ability of this species to belong to two types of cyst-based genera (spiniferate and impagidinioid) suggests that environmental (particularly salinity) and not genetic factors explain the formation of both morphotypes by Gonyaulax baltica, which provides evidence for Heterospory in this species. The affiliation to Gonyaulax baltica demonstrates that Impagidinium caspienense is not endemic to the Caspian Sea. The phylogenetic position of several other gonyaulacoid species is also documented: Impagidinium pallidum, Ataxiodinium choane, Pyxidinopsis psilata, Spiniferites belerius, and Spiniferites ramosus. The LSU and SSU rDNA based phylogenies suggest that the genera Impagidinium and Spiniferites are not monophyletic, and that Pyxidinopsis psilata and Ataxiodinium choane are close to Gonyaulax verior and Gonyaulax polygramma, respectively. In addition, this study accentuates the importance of cyst morphology in the classification of the Gonyaulacales. This article is protected by copyright. All rights reserved.
Muriel Fairon-demaret - One of the best experts on this subject based on the ideXlab platform.
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The significance of Runcaria (Middle Devonian, Belgium) in the evolution of seed plants
Carnets de Géologie, 2005Co-Authors: Philippe Gerrienne, Brigitte Meyer-berthaud, Muriel Fairon-demaretAbstract:The advent of reproduction by seeds was one of the most essential evolutionary steps in plant history: the vast majority of living plants are seed plants (spermatophytes). The seed habit includes the following set of defining characteristics: (1) Heterospory, (2) occurrence of a single megaspore that germinates within an indehiscent megasporangium (nucellus) retained on the sporophyte, (3) enclosure of the megasporangium in an integument, and (4) capture of pollen before seed dispersal. Contrasting hypotheses about the single / multiple, saltational / gradual origin of the seed habit, and identification of the closest relatives of seed plants (aneurophytalean or archaeopteridalean progymnosperms) are still matters of considerable debate. Early seeds did not possess the whole set of characters that define modern seeds. They lacked a true micropyle and an entire integument, and none has yet been discovered containing an embryo. Hence they are called preovules. A suite of characters observed in the Late Devonian preovules Moresnetia and Elkinsia and in a number of younger taxa define hydrasperman reproduction. In hydrasperman preovules, prepollen capture was realized by the modified apex of the nucellus. Prepollen were directed into a hollow lagenostome, then retained in a pollen chamber. A central column attached to the pollen chamber floor sealed the chamber to provide optimal conditions for prepollen germination. The hydrasperman syndrome has been presented as shared by all basal members of the seed plant clade. Discovery of the Lower Carboniferous preovule Coumiasperma challenged this view. Galtier and Rowe (1989, 1991) suggested that the possession of a massive nucellar tip and the lack of pollen chamber characterized another primitive seed organization that either preceded and was ancestral to the hydrasperman type, or evolved independently and was adaptated to wet habitats.
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The significance of Runcaria (Middle Devonian, Belgium) in the evolution of seed plants
Carnets de Geologie, 2005Co-Authors: Philippe Gerrienne, Brigitte Meyer-berthaud, Muriel Fairon-demaretAbstract:The advent of reproduction by seeds was one of the most essential evolutionary steps in plant history: the vast majority of living plants are seed plants (spermatophytes). The seed habit includes the following set of defining characteristics: (1) Heterospory, (2) occurrence of a single megaspore that germinates within an indehiscent megasporangium (nucellus) retained on the sporophyte, (3) enclosure of the megasporangium in an integument, and (4) capture of pollen before seed dispersal (Rothwell & Scheckler, 1988; Haig & Westoby, 1989). Contrasting hypotheses about the single / multiple, saltational / gradual origin of the seed habit (Chaloner, 1989; Galtier & Rowe, 1989; DiMichele et alii, 1989; Bateman & DiMichele, 1994), and identification of the closest relatives of seed plants (aneurophytalean or archaeopteridalean progymnosperms) (Rothwell & Erwin, 1987; Rothwell & Serbet, 1994; Marshall & Hemsley, 2003) are still matters of considerable debate. Early seeds did not possess the whole set of characters that define modern seeds. They lacked a true micropyle and an entire integument, and none has yet been discovered containing an embryo. Hence they are called preovules. A suite of characters observed in the Late Devonian preovules Moresnetia and Elkinsia and in a number of younger taxa define hydrasperman reproduction (Rothwell & Scheckler, 1988). In hydrasperman preovules, prepollen capture was realized by the modified apex of the nucellus. Prepollen were directed into a hollow lagenostome, then retained in a pollen chamber. A central column attached to the pollen chamber floor sealed the chamber to provide optimal conditions for prepollen germination. The hydrasperman syndrome has been presented as shared by all basal members of the seed plant clade (Rothwell & Scheckler, 1988). Discovery of the Lower Carboniferous preovule Coumiasperma challenged this view. Galtier and Rowe (1989, 1991) suggested that the possession of a massive nucellar tip and the lack of pollen chamber characterized another primitive seed organization that either preceded and was ancestral to the hydrasperman type, or evolved independently and was adaptated to wet habitats. At least six different types of Late Devonian (around 365 Ma) preovules or preovule-like structures are known (Prestianni, this volume). This diversity strongly suggests an earlier evolution of the seed habit. The Middle Devonian (around 385 Ma) Runcaria heinzelinii Stockmans 1968 has been recently redescribed on the basis of new specimens, and interpreted as a precursor of seed plants (Gerrienne et alii, 2004). Since then, more new specimens have been isolated from the original fossiliferous slab. Some of them are illustrated here. Hundreds of Runcaria preovules occur in a single large slab of sandstone from the Bois de Bordeaux Formation collected at the "Plan incliné de Ronquières" locality (Belgium) (Stockmans, 1968). Data regarding the age of the locality are numerous. All plant horizons at Plan incliné de Ronquières belong to the upper (but not the uppermost) part of the Bois de Bordeaux Formation, which from conodont evidence is regarded as Givetian or earliest Frasnian (Bultynck, 1991). Other plant remains collected from the same locality include the progymnosperm genera Protopteridium (= Rellimia) and Svalbardia and the cladoxylopsids Pseudosporochnus and Calamophyton (Stockmans, 1968). The concurrent range of those four genera is restricted to the Givetian (Edwards et alii, 2000). Runcaria is found in association with axes and dispersed leaves of the lycopsid genus Leclercqia, that ranges from late Emsian to late Givetian (Meyer-Berthaud et alii, 2003). Finally, palynological samples yielded more than 30 species of trilete micro- and megaspores that constrain a middle to late Givetian age (TA Oppel Zone; 385 Ma) (Gerrienne et alii, 2004). Specimens of Runcaria consist of 6.5 to 8 mm long cupulate preovules (Pl. 1, fig. 1) borne singly at the tip of small axes bifurcating at 40° to 70° (Pl. 1, fig. 2). These stalks do not exceed 3 mm in length and 0.4 mm in diameter. Each preovule is radially symmetrical (Pl. 1, fig. 3). Cupules form a short cup supporting a single preovule. They comprise four to five segments (Pl. 1, fig. 4) free from each other except at the base, each segment ranging from 1.6 to 3 mm in length and 1.1 to 1.7 mm in width. Cupule segments divide in two halves; each half separates distally in two acute tips (Pl. 1, fig. 5). Lateral edges of the cupule segments are decurrent and form rims along the stalk. Preovules consist of a radially symmetrical megasporangium surrounded by a 4.7 to 5.2 mm long laciniate integument. The megasporangium comprises a sessile oval body bearing a distal extension that emerges above the integument and ends up in an enlarged head. The columnar extension is 4 to 5.2 mm long and 0.2 to 0.4 mm wide; the proximal oval body, 1.4 to 2 mm long and 0.9 to 1.3 mm wide. A faint oval mark in the oval body of some specimens may represent the imprint of a megaspore or a megagametophyte, two structures that cannot confidently be observed in ovules preserved as adpressions. The junction between the oval body and the distal extension does not show any bulge that would suggest the occurrence of a pollen chamber. The columnar extension is probably hollow and may be locally enlarged. Its surface is covered by longitudinal ribs separated by deep furrows and twisted counterclockwise. In contrast, the oval body shows a faint polygonal pattern on its surface (Pl. 1, fig. 6). The distal head ending the columnar extension is up to 470 µm wide and 780 µm long. It is made up of cells with thinner walls than those of other cells of the megasporangium. Its external surface is smooth and may show a distal projection. There is no evidence of an opening or dehiscence mark on any part of the megasporangium. The integument separates into a minimum of 16 lobes free from each other and from the megasporangium down to the base. The lobes converge distally, their tips appressed or wrapping around the columnar extension at about 2/3 of its length. Lobes either lie vertically along the megasporangium or separate from it in a counterclockwise spiral, to form the loose walls of a chamber surrounding the basal part of the megasporangium. Individual lobes are up to 100 µm wide and are comprised of elongate thickened cells. In the chamber, they are separated from each other by spaces exceeding 100 µm in width. Many microspores occur around the cupulate preovules and among the integumentary lobes, but none adhere to the distal head of the megasporangium or to the column. Despite the lack of evidence concerning its megaspore/megagametophyte content, Runcaria displays three characters (modified megasporangial apex, dissected integument, and a cupule) specific to early seed plants, that indicate its spermatophytic affinities (Rothwell & Scheckler, 1988). Salient features of the nucellar apex of Runcaria are its extensive length, probably hollow structure, lack of opening and an enlarged cellular tip. Based on that morphology, several scenarios are proposed for the pollination of this ancient seed. All assume that the distal extension of the megasporangium played a significant role because of the selective advantage that its length may have provided in the passive capture of airborne prepollen. If the Ronquières specimens were in a pre-pollination state of development, we are faced with the problem encountered in Coumiasperma where antherozoids must penetrate the wall of a closed megasporangium to reach the female gametophyte (Galtier & Rowe, 1989, 1991). The first scenario involves the lysigeneous dissolution of some nucellar cells, better realized at a site where they are thin-walled. This is the case at the enlarged extremity of the columnar extension, but this site occurs at some distance from the megagametophyte within the oval body. We cannot exclude, therefore, that microspores/ antherozoids caught on the nucellar extension reached the oval body and dissolved cells closer to the presumed site of archegonial development. An alternative mechanism to induce cell wall dissolution, also presented for Coumiasperma, involves siphonogamy. However, the earliest evidence for pollen tubes is much later, within the Late Carboniferous callistophytalean pteridosperms (Rothwell, 1972). If the Ronquières preovules were fossilized at a post-pollination stage, the distal head may represent a mass of cells produced to seal the columnar extension, open before pollination occurred. Available specimens of Runcaria do not provide any information relative to the occurrence of what could be interpreted as a hydrasperman pollen chamber. Furthermore, the mechanism proposed for sealing the preovule after pollination in this third scenario differs significantly from that involving a central column in hydrasperman structures (Rothwell & Scheckler, 1988). The nucellar extension of Runcaria thus differs from that of Coumiasperma and that of the hydrasperman preovules. It is currently premature to assume that the runcarian morphology, and the pollination mechanism attached to it, either is ancestral to all younger ovular morphologies and supports the monophyly of the Spermatophytes, or is divergent and represents an extinct group unrelated to Coumiasperma and to the hydrasperman seed plants. However, as has been already realized from younger evidence, the morphology of Runcaria confirms that pollination was an essential factor driving the evolution of early seed plants (Rothwell & Scheckler, 1988; Haig & Westoby, 1989; Bateman & DiMichele, 1994). An unexpected trait of Runcaria is related to the length of the integumentary lobes that leave the distal extremity of the megasporangium free and unprotected. Aerodynamic experiments (Niklas, 1981) showed that integument lobes projecting above the nucellar tip, a feature shared by most early preovules, created as many zones of turbulence around the lagenostome and decreased the probability that prepollen were specifically directed to this structure. Hypotheses predicting a similar morphology in the most primitive preovules were thus faced with the paradox that the development of such an integument appeared counter-selective for pollination, a naked nucellus being more efficient for that function (Haig & Westoby, 1989). With its integumentary lobes leaving the nucellar tip free and thus providing direct access for airborne prepollen, Runcaria sheds some light on the adaptive significance of the integument in earliest seeds. If adaptive, evolution of the integument was not driven by selective forces related to pollination but to other necessities yet to be identified, presumably related to protection, nutrition and/or dispersal. The late Middle Devonian Runcaria is a complex structure that already possessed the full set of attributes, including a cupule, that characterize younger seeds. It shows that the evolution of the integument was potentially involved in protection, nutrition and/or dispersal. The morphology of the nucellar apex of Runcaria differs from that of the hydrasperman preovules which therefore can no longer be presented as the single primitive type in the seed plant stem group. Runcaria evolved in the Givetian when progymnosperms were represented by the Aneurophytales and the earliest representatives of the Archaeopteridales (Beck & Wight, 1988). The Middle Devonian age of Runcaria closes the stratigraphical and evolutionary gap between the Aneurophytales and the oldest seed plants that, prior to this discovery, was presented as a major argument against a direct relationship between these two groups (Hilton, 1998; Marshall & Hemsley, 2003). Moreover, if seed characters were acquired sequentially, it implies an earlier, possibly early Middle Devonian, origin for the seed habit, when the only known representatives of the progymnosperms were aneurophytaleans.
Martin Burd - One of the best experts on this subject based on the ideXlab platform.
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The adaptive value of Heterospory: Evidence from Selaginella.
Evolution, 2018Co-Authors: Kurt B. Petersen, Martin BurdAbstract:Heterospory was a pivotal evolutionary innovation for land plants, but it has never been clear why it evolved. We used the geographic distributions of 114 species of the heterosporous lycophyte Selaginella to explore the functional ecology of microspore and megaspore size, traits that would be correlated with many aspects of a species' regeneration niche. We characterized habitats at a global scale using leaf area index (LAI), a measure of foliage density and thus shading, and net primary productivity (NPP), a measure of growth potential. Microspore size tends to decrease as habitat LAI and NPP increase, a trend that could be related to desiccation resistance or to filtration of wind-borne particles by leaf surfaces. Megaspore size tends to increase among species that inhabit regions of high LAI, but there is an important interaction with NPP. This geographical pattern suggests that larger megaspores provide an establishment advantage in shaded habitats, although in open habitats, where light is less limiting, higher productivity of the environment seems to give an advantage to species with smaller megaspores. These results support previous theoretical arguments that Heterospory was originally an adaptation to the increasing height and density of Devonian vegetative canopies that accompanied the diversification of vascular plants with leaves.
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The enigma of sex allocation in Selaginella.
Annals of Botany, 2017Co-Authors: Kurt B. Petersen, Martin BurdAbstract:Background and Aims The division of resource investment between male and female functions is poorly known for land plants other than angiosperms. The ancient lycophyte genus Selaginella is similar in some ways to angiosperms (in Heterospory and in having sex allocation occur in the sporophyte generation, for example) but lacks the post-fertilization maternal investments that angiosperms make via fruit and seed tissues. One would therefore expect Selaginella to have sex allocation values less female-biased than in flowering plants and closer to the theoretical prediction of equal investment in male and female functions. Nothing is currently known of sex allocation in the genus, so even the simplest predictions have not been tested. Methods Volumetric measurements of microsporangial and megasporangial investment were made in 14 species of Selaginella from four continents. In five of these species the length of the main above-ground axis of each plant was measured to determine whether sex allocation is related to plant size. Key Results Of the 14 species, 13 showed male-biased allocations, often extreme, in population means and among the great majority of individual plants. There was some indication from the five species with axis length measurements that relative male allocation might be related to the release height of spores, but this evidence is preliminary. Conclusions Sex allocation in Selaginella provides a phylogenetic touchstone showing how the innovations of fruit and seed investment in the angiosperm life cycle lead to typically female-biased allocations in that lineage. Moreover, the male bias we found in Selaginella requires an evolutionary explanation. The bias was often greater than what would occur from the mere absence of seed and fruit investments, and thus poses a challenge to sex allocation theory. It is possible that differences between microspores and megaspores in their dispersal ecology create selective effects that favour male-biased sexual allocation. This hypothesis remains tentative.
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Why did Heterospory evolve
Biological Reviews, 2016Co-Authors: Kurt B. Petersen, Martin BurdAbstract:The primitive land plant life cycle featured the production of spores of unimodal size, a condition called homospory. The evolution of bimodal size distributions with small male spores and large female spores, known as Heterospory, was an innovation that occurred repeatedly in the history of land plants. The importance of desiccation-resistant spores for colonization of the land is well known, but the adaptive value of Heterospory has never been well established. It was an addition to a sexual life cycle that already involved male and female gametes. Its role as a precursor to the evolution of seeds has received much attention, but this is an evolutionary consequence of Heterospory that cannot explain the transition from homospory to Heterospory (and the lack of evolutionary reversal from Heterospory to homospory). Enforced outcrossing of gametophytes has often been mentioned in connection to Heterospory, but we review the shortcomings of this argument as an explanation of the selective advantage of Heterospory. Few alternative arguments concerning the selective forces favouring Heterospory have been proposed, a paucity of attention that is surprising given the importance of this innovation in land plant evolution. In this review we highlight two ideas that may lead us to a better understanding of why Heterospory evolved. First, models of optimal resource allocation - an approach that has been used for decades in evolutionary ecology to help understand parental investment and other life-history patterns - suggest that an evolutionary increase in spore size could reach a threshold at which small spores yielding small, sperm-producing gametophytes would return greater fitness per unit of resource investment than would large spores and bisexual gametophytes. With the advent of such microspores, megaspores would evolve under frequency-dependent selection. This argument can account for the appearance of Heterospory in the Devonian, when increasingly tall and complex vegetative communities presented competitive conditions that made large spore size advantageous. Second, Heterospory is analogous in many ways to anisogamy. Indeed, Heterospory is a kind of re-invention of anisogamy within the context of a sporophyte-dominant land plant life cycle. The evolution of anisogamy has been the subject of important theoretical and empirical investigation. Recent work in this area suggests that mate-encounter dynamics set up selective forces that can drive the evolution of anisogamy. We suggest that similar dispersal and mating dynamics could have underlain spore size differentiation. The two approaches offer predictions that are consistent with currently available data but could be tested far more thoroughly. We hope to re-establish attention on this neglected aspect of plant evolutionary biology and suggest some paths for empirical investigation.