Why did heterospory evolve?

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.     

Chloroplast proliferation based on their distribution and arrangement inAdiantum protonemata growing under red light

Chloroplast proliferation was investigated inAdiantum protonemata growing under continuous red light. Cell division is absent when cells are grown under red light. The chloroplast number increases as the cell length increases, therefore the chloroplasts divide in the absence of cell division. Chloroplasts in the basal part of the filamentous protonemal cell migrate gradually toward the cell apex, but there is no large net migration from the tip to the base or vice versa, indicating that chloroplast division takes place in the apical part of the protonemata. Chloroplast number in the apical 100 μm was maintained at about 200 during cell growth at least over eight days. The chloroplasts were either dumbbell- or ellipsoid-shaped. Dumbbell-shaped chloroplasts are abundant everywhere in a protonema, ranging from 30 to 50% of the total chloroplasts. The dumbbell-shaped chloroplasts attached to or very close to the plasma membrane seem to be the ones that are dividing but the dumbbell-shaped ones in the other regions do not divide. These data support the hypothesis that a signal from the plasma membrane induces the dumbbell-shaped chloroplasts to divide.     

Size dependency of gametophytes decay inAthyrium brevifrons Nakai during spring desiccation

Gametophyte populations inAthyrium brevifrons were analysed with respect to population size and surviving area (%) of individual thalli in a transplant garden at Sapporo during 5–26 April 1983, to study the safe-microsite for gametophyte establishment in nature. Spores dispersed in August 1982 germinated and grew into thalli of various widths (<10 mm); 10.3% of the thalli matured by early October 1982. Maturation was attained by gametophytes of width 4–7 mm. The number of gametophytes gradually decreased with increasing width. By April 1983, 20.5% of total gametophytes were mature with a mode of 5–6 mm in width. The relative number of gametophytes with surviving area of 2–20% increased and that of 85–100% decreased in accordance with collection days delayed until after snow-melt. Surviving area (%) on gametophyte of all widths decreased with decreasing soil moisture contents. In particular, immature gametophytes of 2–4 mm width showed a significant correlation (P<0.01) between soil moisture content and relative number of gametophytes with 0–20% surviving area and mean surviving area (%) of every width of thalli. The spring desiccation might be a factor that reduces or limits gametophyte populations in nature.     

Involvement of colchicine-sensitive cytoplasmic element in premitotic nuclear positioning ofAdiantum protonemata

Summary When the red-light grown protonema ofAdiantum capillus-veneris was transferred to the dark, the nucleus ceased its migration ca. 5 hours before cell plate formation (Mineyuki andFuruya 1980). To see whether the nucleus was held by some cytoplasmic structure during nuclear positioning, protonemata were treated with various centrifugal forces at different stages of the cell cycle. Nuclei of G₁ phase were easily displaced by centrifugation at 360×g for 15 minutes, but those of G₂ or M phase were not displaced by it, suggesting that the nuclei were held by some cytoplasmic elements in G₂ or M phase. This nuclear anchoring was not detectable in protonemata that were treated with 5mM colchicine. With this treatment, the nucleus did not stop its migration at late G₂ and moved even in prophase. And the retardation of organelle movement which was observed in cytoplasm on the lateral side of the nucleus after the cessation of premitotic nuclear migration (Mineyuki andFuruya 1984) was not observed in the presence of colchicine. Thus the nuclei appear to be held by colchicine-sensitive structure in cytoplasm between the lateral surface of the nucleus and cell wall during the premitotic nuclear positioning. Electron micrographs showing cytoplasmic microtubules were consistent with the idea.Abbreviations PPN Premitotic positioning of the nucleus - L region Cytoplasm between the lateral surface of the nucleus and cell wall (seeMineyuki et al. 1984)     

红皮树胚胎发育

本文报道红皮树(Styrax suberifoltus Hook.et Arn.)大小孢子发育和早期胚胎发生。子房具胚珠20—23枚,胚珠横生,珠被二层,薄珠心,孢原细胞直接起大孢子母细胞作用。合点端大孢子具功能。胚囊发育为正常型。成熟胚囊具大量淀粉粒。小孢子形成为同时型,成熟花粉为二细胞型。传粉后、受精前两个助细胞在形状和对苏木精着色程度上有显著区别。胚乳发育为细胞型。在合子分裂前,胚乳细胞增至约26个时,暂时停止分裂。苏木精对细胞质不易着色,似解体细胞。有胚乳吸器。     

Ecological optima and tolerances of Thelypteris limbosperma,Athyrium distentifolium,and Matteuccia struthiopteris along environmental gradients in Western Norway

The distribution and abundance of Thelypteris limbosperma, Athyrium distentifolium, and Matteuccia struthiopteris are modelled statistically in relation to 14 environmental variables along the major climatic, topographic, and edaphic gradients in western Norway. The data are from 624 stands from which measurements or estimates of mean January and mean July temperatures, humidity, altitude, aspect, and slope are available. From 182 of these stands eight soil variables have also been measured. The species responses are quantified by two numerical methods: Gaussian logit regression and weighted averaging (WA) regression. The estimated WA optima suggest that A. distentifolium has an ecological preference for low July and January temperatures, high altitudes, and soils of low-medium pH and base content. The species shows statistically significant Gaussian responses with summer temperature, humidity (= Martonnes humidity index), altitude, slope, aspect, pH, cation exchange capacity, and base saturation with optima of 8.7 °C, 188.9, 1220 m, 28°, 29°, 4.8, 13.77 mEq 100 g dry soil^-1, and 13.4%, respectively. These suggest that the occurrence and relative abundance of A. distentifolium are well predicted by summer temperature, topography, and soil pH and base status. T. limbosperma has WA optima that suggest that it favours moderately high winter and summer temperatures, high humidity, medium altitude, and soils of low pH and base content. It has significant Gaussian responses to summer temperature (optimum =12.6 °C), winter temperature (-1.8 °C), humidity (179.2), altitude (459.5 m), slope (22.5°), and Na (0.7 mg 100 g dry soil^-1). These suggest that climatic factors, altitude, and slope are significant predictors for its occurrence and abundance. M. struthiopteris has high WA optima for summer temperature, pH, Ca, Mg, K, Na, cation exchange capacity (CEC), and base saturation, and a low optima for humidity and winter temperature. Of these, summer temperature (16.0 °C), Ca (63.1 mg 100 g dry soil^-1), Mg (41.0 mg 100 g dry soil^-1), K (23.6 mg 100 g dry soil^-1), Na (5.0 mg 100 g dry soil^-1), CEC (60.7 mEq 100 g dry soil^-1), and base saturation (56.3%) have significant Gaussian logit responses, as do aspect (150.2°) and loss-on-ignition (9.4%). These results suggest that the occurrence and relative abundance of M. struthiopteris are well predicted by high soil base cations, a generally southern aspect, low organic content in the soil, and high July temperatures.     

ISOLATION AND CHARACTERIZATION OF PHASE-SPECIFIC COMPLEMENTARY DNAs FROM SPOROPHYTES AND GAMETOPHYTES OF PORPHYRA PURPUREA (RHODOPHYTA) USING SUBTRACTED COMPLEMENTARY DNA LIBRARIES1

The red alga Porphyra purpurea (Roth) C. Agardh has a life cycle that alternates between shell-boring, filamentous sporophytes and free-living, foliose gametophytes. The significant morphological differences between these two phases suggest that many genes should be developmentally regulated and expressed in a phase-specific manner. In this study, we prepared and screened subtracted complementary DNA (cDNA) libraries specific for the sporophyte and gametophyte of P. purpurea. This involved the construction of cDNA libraries from each phase, followed by the removal of common clones through subtractive hybridization. Sampling of the subtracted libraries indicated that 8–10% of the recombinant colonies in each library were specific for the appropriate phase. Of 20 putative phase-specific cDNAs selected from each subtracted library, eight unique clones were obtained for the sporophyte and seven for the gametophyte. After confirming their phase-specificities by hybridization to gametophyte and sporophyte messenger RNA, these 15 phase-specific cDNAs were sequenced, and the deduced amino acid sequences were used to search protein databanks. Two proteins encoded by the sporophyte-specific cDNAs and two by the gametophyte-specific cDNAs were identified by their similarity to databank entries.     

Isolation and culture of protoplasts from young sporophytes ofSalvinia natans aseptically obtained by co-culture of female and male gametophytes

Sporophytes were aseptically obtained by co-culture of female and male gametophytes derived from two types of spores (megaspores and microspores) of the heterosporous fernSalvinia natans All. Protoplasts isolated enzymatically from juvenile leaflets of sporophytes were cultured in a 1/10 Murashige and Skoog's medium containing 2.2 M naphthalene acetic acid, 2.2 M 6-benzyl-aminopurine, 0.35 M mannitol, and 0.05 M sucrose. Cell division took place within 6 days of culture, and cell-clusters composed of 9–10 cells were observed after 30 days of culture.Abbreviations BA 6-benzyl-aminopurine - MS Murashige and Skoog - NAA naphthaleneacetic acid     

Phytochrome-mediated phototropism inAdiantum cuneatum young leaves

Phototropism of youngAdiantum fern leaves is induced by red light as well as blue light. The red light response is mediated by phytochrome. This is the first evidence of phytochrome action in diploid fern tissue. The blue light response is mainly mediated not by phytochrome, but probably by a blue light-absorbing pigment as in the case of almost all plants and fungi. The red light-induced phototropism becomes detectable within 2 hr after the onset of unilateral light. The highest bending rate is about 10 degrees/hr, which occurs between 3–5 hr after the induction of the tropic response. The bending region is about 6–8 mm from the highest point of the coiled crozier where the growth rate becomes slow.     

马蹄香大小孢子发生及雌雄配子体形成

马蹄香（Ｓａｒｕｍａｈｅｎｒｙｉ,Ｏｌｉｖ．）花药壁的发育属双子叶型。花粉母细胞减数分裂为同时型,四分体主要为四面体形,少数为左右对称式排列。腺质绒毡层,其细胞可排列为不规则的两层,双核或多核。到单细胞花粉阶段,绒毡层细胞内切向壁上出现许多乌氏体。成熟花粉为２细胞型,圆球状,具单萌发沟。雌蕊６心皮,上部彼此分离、下部联合。倒生胚珠,双珠被,厚球心。胚囊发育蓼型。成熟胚囊为七细胞结构,但两个助细胞退化较早。