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.     

中国产桫椤类配子体在不同基质胁迫下的形态畸变多样性研究

为了明确土壤污染对中国产桫椤科植物的生殖影响,以探讨中国桫椤类乃至其他濒危蕨类濒危的内在原因。该研究以中国产9种桫椤植物为材料,采用各植物原产地地表土壤培养孢子至配子体成熟,全程比较观察配子体畸变类型,统计畸变率;并分别取3个属的代表种笔筒树、大叶黑桫椤和中华桫椤的孢子,以MS培养基为对照,分别用5mg·kg^-1Pb^2+和1mg·kg^-1Cd^2+单因素胁迫培养配子体,比较观察各组的配子体形态畸变类型,统计畸变率。结果表明:(1)中国产桫椤类9种植物配子体培养过程中共出现17种形态畸变类型,其中4种为分化畸变,7种为假根畸变,4种为细胞畸变,2种为性器畸变。(2)不同培养基——对照组(MS培养基)、原产地土壤、Cd^2+胁迫、Pb^2+胁迫对中国产桫椤科9种植物配子体的平均畸变比例分别是2.28%、12.61%、31.58%和33.58%。(3)9种桫椤的配子体畸变类型及其畸变率均与培养基的胁迫程度呈正相关关系,其配子体发育都受到了产地土壤污染的严重伤害。(4)3种桫椤属植物配子体对土壤环境胁迫的耐受力强弱为黑桫椤属>白桫椤属>桫椤属。(5)桫椤科配子体的形态畸变多样性与胁迫条件之间具有稳定的对应关系。     

中国特产2种濒危水韭雄性发育的比较研究

采用透射电镜和光镜比较观察了中华水韭和云贵水韭雄配子体及其精子的发育特征。结果显示:(1)2种水韭的雄配子体的寿命只有15~30d,终生都在小孢子壁内发育。(2)雄配子体只含有1个原叶体细胞、1个精子器壁细胞和4个精细胞,前2个细胞内含有丰富的营养物质。(3)精子由精核、微管带、鞭毛、细胞质等4部分构成。(4)中华水韭雄配子体发生率为4.5%,平均每个雄配子体产生0.46个精子,精子游动速度约53μm/s,寿命8min;云贵水韭雄配子体发生率和产精量略高于中华水韭,但精子游动速度和寿命略低于中华水韭。研究认为,中国水韭濒危的主要原因之一是雄配子体产精率低、受到生殖生态限制、水污染对精子的危害等;雄性特征表明水韭在石松类中占有较高的进化地位;结合前人的研究成果绘出了水韭雄配子体的发育模式。     

龙津蕨的配子体发育特征及其系统学意义

采用改良Knop's营养液液体培养基于25℃恒温培养箱中培养龙津蕨(Mesopteris tonkinensis)的孢子,每天光照12 h,黑暗12 h,光照强度为2 500 lx。用光学显微镜观察记录其孢子萌发、配子体发育的全过程,为龙津蕨系统学的研究提供配子体发育方面的详实资料。结果表明:成熟孢子深褐色,不透明,两侧对称,极面观椭圆形,赤道面观圆角三角形,具单裂缝,孢子周壁具密集的脊状褶皱。播种后15 d左右萌发,形成2~5个细胞长的丝状体。孢子萌发类型为书带蕨型(vittaria-type)。具单细胞假根,不含叶绿体,基部膨大。20 d左右发育成片状体,30 d左右形成幼原叶体,幼原叶体不对称,成熟原叶体心脏形对称,56 d左右形成成熟原叶体,原叶体发育类型为铁线蕨型(adiantum-type)。幼原叶体仅左右两翼顶端细胞产生乳突毛状体,成熟原叶体边缘及背腹面都具少量乳突毛状体,毛状体由单细胞构成。68 d左右精子器开始出现,精子器近圆球形,由基细胞、环细胞、盖细胞构成。75 d左右颈卵器出现,成熟颈卵器颈部由3层细胞构成。其侧面观柱状,顶面观为铜面状。颈卵器垂直于原叶体表或向原叶体基部倾斜。另外,根据已知的金星蕨科其他属的配子体发育特征,发现龙津蕨配子体发育的这些特征与他们存在较大的区别,因此龙津蕨系统学位置还有待于进一步研究。     

Evolution and maintenance of haploid–diploid life cycles in natural populations: The case of the marine brown alga Ectocarpus

The evolutionary stability of haploid–diploid life cycles is still controversial. Mathematical models indicate that niche differences between ploidy phases may be a necessary condition for the evolution and maintenance of these life cycles. Nevertheless, experimental support for this prediction remains elusive. In the present work, we explored this hypothesis in natural populations of the brown alga Ectocarpus. Consistent with the life cycle described in culture, Ectocarpus crouaniorum in NW France and E. siliculosus in SW Italy exhibited an alternation between haploid gametophytes and diploid sporophytes. Our field data invalidated, however, the long‐standing view of an isomorphic alternation of generations. Gametophytes and sporophytes displayed marked differences in size and, conforming to theoretical predictions, occupied different spatiotemporal niches. Gametophytes were found almost exclusively on the alga Scytosiphon lomentaria during spring whereas sporophytes were present year‐round on abiotic substrata. Paradoxically, E. siliculosus in NW France exhibited similar habitat usage despite the absence of alternation of ploidy phases. Diploid sporophytes grew both epilithically and epiphytically, and this mainly asexual population gained the same ecological advantage postulated for haploid–diploid populations. Consequently, an ecological interpretation of the niche differences between haploid and diploid individuals does not seem to satisfactorily explain the evolution of the Ectocarpus life cycle.     

GENETIC POPULATION STRUCTURE AND MATING SYSTEM IN CHONDRUS CRISPUS (RHODOPHYTA)1

Chondrus crispus Stackh. has been intensely studied, yet no study to date has elucidated its population structure or mating system despite many populations in which there was a haploid bias and lack of male gametophytes. Therefore, 12 nuclear microsatellite loci were identified in this red alga. Microsatellite markers were developed and tested against a panel of specimens collected from two shore levels at two sites in Brittany, France: Pointe de Primel and Pointe de la Jument, Concarneau. Single locus genetic determinism was verified at eight polymorphic loci, as only one band was observed for haploid genotypes, whereas one or two bands were observed for diploids. These markers enabled the detection of unique genotypes within sampled populations, indicating that very few fronds shared the same multilocus genotype. This finding suggests that asexual reproduction was not the prevailing mode of reproduction. In addition, we explored the hierarchical population structure showing that gene flow is restricted at small spatial scales (<50 m) between upper and lower Chondrus‐range populations within a shore. Sexual reproduction predominated in the populations of C. crispus studied, but probably due to fine‐scale spatial substructuring, inbreeding was also significant. In conclusion, this study reveals that fine‐scale genetic variation is of major importance in C. crispus, suggesting that differences between microhabitats should be essential in understanding evolutionary processes in this species.     

不同植物生长调节物质对细叶小羽藓愈伤组织诱导及分化的影晌

以细叶小羽藓[Haplocladium microphyllum（Hedw．）Broth．】的配子体为外植体,研究了不同植物生长调节物质对细叶小羽藓愈伤组织诱导及分化的影响。结果表明：在Ms培养基中添加不同的植物生长调节物质对细叶小羽藓配子体增殖影响差异很大,具体表现为2,4一D、KT、IBA和IAA促进细叶小羽藓芽体的诱导及生长,NAA抑制细叶小羽藓芽体的诱导,TDZ阿姨、、及6－BA促进细叶小羽藓的配子体产生愈伤组织;在Ms培养基中添加0．3mg·L－1。6-BA最适合愈伤组织的诱导;在MS掊养基中添加1．0mg·L－1。。IBA最适合愈伤组织的分化。     

Study of the independent gametophytes found on Jeju Island in South Korea and the first record of the obligate independent gametophyte of Antrophyum obovatum Baker

Fern gametophytes have often been neglected in research; however, studies on gametophytes are crucial for a better understanding of the evolution of ferns. During their life cycle, some gametophytes produce large and long‐lived populations without producing sporophytes and reproduce independently through asexual means, such as through the formation of gemmae. In this study, we investigated independent gametophytes on the Jeju Island of Korea, which was located on the land bridge between East China and Japan during the glacial periods. Fourteen gametophyte populations were collected from seven sites, of which 13 populations were clearly identified as belonging to four fern species known to occur in Jeju Island with BLAST searches using rbcL and trnL‐F sequences. Surprisingly, the last remaining population constituted undescribed taxa in Korea. We presented the first report of the independent gametophytes of Antrophyum obovatum Baker which has not been previously recorded in Korea. It has been supposed that many ferns sought suitable habitat throughout the land bridge between China and Japan. However, Jeju Island might be unsuitable for vittarioid ferns that prefer a tropical or subtropical environment. Consequently, only two species of vittariod ferns (A. obovatum and Haplopteris flexuosa (Fée) E.H. Crane) in the form of a gametophyte and sporophyte, respectively, exist on Jeju Island. Therefore, this gametophyte population must be protected and managed from a conservation perspective. In the case of the independent gametophyte of Hymenophyllum wrightii Bosch, haplotype analysis was conducted based on the rbcL sequences and the result supported that the North American populations were migrated from Japan through land bridge during the glacial periods and Jeju populations were recently established by long‐distance dispersal of the Japanese populations.     

竹叶蕨配子体发育的培养观察

首次在光学显微镜下观察竹叶蕨孢子及其萌发、丝状体发育、片状体和生长点的形成及分化、原叶体细胞形态、假根及性器的发育等方面所表现出的显微特征。初步讨论竹叶蕨科从鳞始蕨科中分立出来的合理性,以及原叶体边缘细胞的形态、叶绿体对光的敏感性、假根的形态和精子器的形成及分化的系统学意义。     

Bridging the generation gap: flowering plant gametophytes and animal germlines reveal unexpected similarities

Alternation of generations underpins all plant life histories and is held to possess important adaptive features. A wide range of data have accumulated over the past century which suggest that alternation from sporophyte to gametophyte in angiosperms includes a significant phase of 'informational reprogramming', leaving the founder cells of the gametophyte developmentally uncommitted. This review attempts to bring together results from these historic studies with more recent data on molecular and epigenetic events which accompany alternation, gametophyte development and gametogenesis in angiosperms. It is striking that most members of the other principal group of multicellular eukaryotes – the animals - have a completely different a life history: animals generate their gametes directly from diploid germlines, often set aside early in development. Nevertheless, a comparison between animal germlines and angiosperm gametophyte development reveals a number of surprising similarities at the cytological and molecular levels. This difference in life history but similarity in developmental process is reviewed in the context of the very different life strategies adopted by plants and animals, and particularly the fact that plants do not set aside diploid germlines early in development.