种子植物的起源及其早期演化

1、植 物 学 报 1998 , 40 (11) :981987act a bot a nica si nicarevie w种子植物的起源及其早期演化j aso n hilto n( 中国科学院植物研究所 北京 100093)摘要 迄今为止世界上最早的种子植物发现于北美和欧洲西北部的晚泥盆纪地层 。这些种子植物可能起源于前裸子植物 ,即解剖构造上较为进化的一类蕨类植物 。然而 ,从目前的证据来看尚无法确切判断 这些种子植物起源于前裸子植物中的某个特定类群 ,比如古羊齿目和戟枝木目 。作者总结了种子植物 祖先的有关证据 ,并对目前已知的最早的种子植物的形态进行了概述 。由于种子植物自出现起即表现

2、出丰富的形态多样性 ,因而尚无法对某个可能的祖先形态作出判断 。目前的研究尚不能确切回答种子 植物起源的全部问题 ,仍需寻找有关前裸子植物与种子植物过渡类型的新证据 。作者对未来的研究提 出了建议 。关键词 种子植物 , lignop hyte , 前裸子植物 , 演化 , 系统发育review of the fossil evid ence fo r the o ri gi na nd ea rl iest evol utio n of the seed2pl a nts j aso n hilto n ( depa rt ment of pal aeobot a ny , i nst i

3、t ute of b ot a ny , t he chi nese a ca de m y of s ciences , beijing 100093)the earliest identified seed2plant s are found in latest devo nian aged sediment s f ro mabstractnort hwest europe and nort h america and are believed to have evolved f ro m wit hin t he p rogym2 nosper ms , a group of anat

4、o mically advanced pteridop hytic plant s. however , current evidence makes it p roblematic to deter mine f ro m which particular p rogymnosper mous lineage t he seed2plant s e2 volved , wit h t he major co ntender being t he aneurop hytalean p rogymnosper ms. the evidence for t he ancest ral stock

5、to t he seed2plant s is summarised and t he morp hologies of t he earliest known seed2 plant s are co nsidered. fro m t heir first geological occurrence , t he seed2plant s are morp hologically di2 verse to such an extent t hat t he identificatio n of a single potential ancest ral morp hology is imp

6、ossible . in t he light of t he evidence so far p resented , t he p recise origin of t he seed2plant s is unresolved and in need of new evidence relating to t he p rogymnosper m/ seed2plant t ransitio n . fut ure lines of research are also suggested.key words sper matop hyte , lignop hyte , progymno

7、sper m , evolutio n , phylogenythe seed2plant s co nstit ute a significant co mpo nent of t he modern flora and for m t he basis of eco no mic plant resources such as agricult ural crop s and timber . despite t he obvious importance oft his plant group , t here are still many important questio ns ab

8、out t heir origin and evolutio nary rela2tio nship s (p hylogeny) which remain largely unresolved , or for which o nly recent research has p ro2 vided suitable answers. for example , o nly wit hin t he past decade or so has t he co nsensus of opinio n been t hat t he group evolved f ro m a single ev

9、olutio nary event , wit h p revious co ncept s co nsidering t hem to have evolved f ro m t wo (or more) separate event s. this p revious interp retatio n viewed t he project suppo rted by t he chinese academy of sciences special fund of biological sciences and technology , ( t he spe2cial item of t

10、he minist ry of finance , china) s tz21201 . co rrespo nding address : depart ment of eart h sciences , u niversit y of wales cardiff , p o bo x 914 , cardiff , cf13 ye ,u k.received :1998203224 revised :1998208204group to be“nat urally”split bet ween separate orders , alt hough t he p recise delimi

11、tatio n of t hese wasof ten argued1 5 . the most co nvincing evidence for a single origin to t he seed2plant s co mes not f ro m t he st udy of t he living members of t he group , but f ro m t he fossil record. the earliest seed2plant s are now known to have a co mmo n type of rep roductive biology

12、ter med“hydrasper man rep ro2ductio n”6 ,7 . this t ype of rep roductio n is characterised by an elaborate pollen chamber (for pollenreceptio n and retentio n p rior to fertilisatio n) in which t he initial develop ment of t he ovule post2polli2 natio n seals t he opening to t he pollen chamber6 . r

13、ot hwell6 deter mined t hat t his kind of elaborate sealing mechanism was unlikely to have evolved more t han o nce in seed2plant evolutio n , and t hat all ot her t ypes of pollen chamber are subsequently derived f ro m it . this suggest s t hat t he seed2plant s are mo nop hyletic , evolving o nly

14、 o nce and for ming a nat ural evolutio nary group . this mo nop hyletic sta2t us necessitates all seed2plant s to be placed in a single higher taxo n , namely t he divisio n sper mato2p hyta8 .the fossil record of t he sper matop hytes dates back into t he famennian stage of t he late devo2nian , a

15、pp ro ximately 365 millio n years ago 7 9 . fossil sper matop hytes of t his age have been identi2fied f ro m part s of nort h2east europe and nort h america alt hough , t he possibilit y exist s for bot h old2 er fossil sper matop hytes or new fossil finds f ro m ot her geograp hical regio ns. due

16、to f requent f rag2 mentatio n of plant s p rior to fossilisatio n , entire plant s are rarely found as fossils. in t he case of t he earliest sper matop hytes , it is of ten t he case t hat o nly t he ovules and seed t hemselves ( t he fertilised and unfertilised equivalent s) are identifiable as u

17、nequivocal sper matop hytes ; during t his early stage oft heir evolutio n t hey shared t he majorit y of t heir ot her morp hological and anato mical characters wit h t heir immediate ancestors , t he p rogymnosper ms9 . recent investigatio ns show t hat t he earliestsper matop hytes (of late devo

18、nian age) are neit her rare nor morp hologically limited9 15 as p revi2ously t hought . in fact , f ro m t heir first geological occurrence , t hey co nstit ute a major part of t he flo2ral assemblages which co ntain t hem9 .st udies of extant plant s have so far p rovided lit tle or no infor matio

19、n co ncerning t he origin and early radiatio n of t he sper matop hytes and details of t hese important early stages in sper matop hyte evolutio n have o nly co me f ro m st udies of fossil plant s. however , t hese event s in early sper mato2 p hytes history are of co nsiderable importance to p hyl

20、ogenetic st udies of t he group as a whole ; t he taxo no my , systematics and evolutio nary relatio nship s of t he living ( crown) sper matop hytes depend to a large extent o n t he relatio nship s bet ween t heir ancest ral ( stem) group s. thus , understanding of fossil sper matop hytes is an es

21、sential co mpo nent in t he co ntinued develop ment of co ncept s relating to sper matop hyte origin , systematics , taxo no my , evolutio n and p hylogeny.1a ncestral stoc k of the spermatop hy tescurrent evidence leads us to suspect t hat t he sper matop hytes evolved f ro m wit hin t he p rogym2n

22、osper ms , an advanced group of devo nian and carbo niferous pteridop hytes which t hemselves evolvedf ro m wit hin t he devo nian trimerop hytes16 ,17. progymnosper ms are characterised by t he co mbina2tio n of pteridop hytic rep roductio n ( bot hho mosporous and heterosporous st rategies) and “g

23、ym2nosper mous” stem anato my wit h seco ndary xylem characterised by bifacial vascular cambi2um2 ,3 ,18 21 . before t he discovery of t he p rogymnosper ms18 21 t his kind of seco ndary wood wast hought to be rest ricted to t he seed2plant s alo ne . thus , t he p rogymnosper ms are rep roductively

24、 p rimitive but anato mically advanced. st ratigrap hically , t he oldest p rogymnosper ms ( aneurop hy2 tales) are of middle devo nian age wit h o nly a few members of t he group persisting into t he carbo nif2 erous , wit h t he notable exceptio n of t he upper carbo niferous p utative p rogymnosp

25、er m cecrop2 sis 22 .the earliest seed2plant s of famennian and tournaisian age have stem anato my similar to t he aneurop hytalean p rogymnosper ms23 , leading to assumptio ns t hat t he sper matop hytes evolved f ro m wit hin t his p rogymnosper m order7 . however , current evidence indicates t ha

26、t t he aneurop hytalean11 期j aso n hilto n : 种子植物的起源及其早期演化 (英)983p rogymnosper ms became extinct at or near t he end of t he middle devo nian21 , p resenting a major st ratigrap hic gap bet ween t he p resumed ancestors and t heir assumed descendant s. furt her more , aneurop hytales are rep roducti

27、vely p rimitive , being ho mosporous and seemingly lacking unequivocal heterospory24 . if t his ancestor descendant relatio nship is correct , as suggested by stem anato my , it suggest s t hat older sper matop hytes and/ or younger aneurop hytes may exist in t he fossil record. clearly , new finds

28、of relevant fossils f ro m t his time interval are needed to resolve t his important issue . in synt hesis , t here are no currently identified plant s which are morp hologically and st rati2 grap hically inter mediate bet ween t he p rogymnosper ms and t he sper matop hytes.at and immediately p rio

29、r to t he first st ratigrap hic occurrence of t he sper matop hytes in t he latest devo nian , t he archaeopteridalean p rogymnosper ms were bot h numerous in fossil assemblages and geograp hically widesp read2 ,19 ,21 ,25 . archaeopteridalean p rogymnosper ms , unlike t he aneurop hy2 tales , inclu

30、ded rep roductively advanced heterosporous plant s such as t he genus a rchaeopteris it self 21 . for many researchers t he p resence of heterospory in co mbinatio n wit h t heir co ntemporane2 ous st ratigrap hical occurrence wit h t he earliest sper matop hytes has suggested t hat t he sper matop

31、hytes may have evolved f ro m wit hin t he archaeopteridales2 ,25 .recent p hylogenetic analyses of t he p rogymnosper ms and sper matop hytes ( collectively ter medlignop hytes) has so far failed to adequately resolve t he p roblems of sper matop hyte origin26 ,27 . these analyses show t he p rogym

32、nosper ms to be a closely related sistergroup to t he sper matop hytes , wit h t he archaeopteridales nesting closer to t he sper matop hytes t han t he aneurop hytales27 . these result s do not directly support or disp rove an origin of t he sper matop hytes f ro m wit hin t he ar2 chaeopteridales

33、, merely indicating t hat t he archaeopteridales are closer in overall morp hology to p rimitive members of t he sper matop hytes (e . g. el ki nsi a) t han are t he currently identified aneuro2 p hytalean taxa . the identificatio n of t he p recise ancest ral stock for t he sper matop hytes is t hu

34、s , unre2solved. it is now necessary to identif y f urt her“advanced”p rogymnosper ms and“p rimitive”sper2matop hytes in order to p rovide new evidence for t his important morp hological t ransitio n and to nar2 row t he existing gap . wit hout t he identificatio n of new fossil materials , t he ori

35、gin of t he sper mato2 p hytes will remain unresolved.2the ea rl iest spermatop hy tes of the l atest d evo nia nfro m t heir earliest occurrence in t he latest devo nian , t he sper matop hytes are morp hologicallydiverse making it impossible to identif y a single ancest ral morp hology f ro m whic

36、h t he ot hers are de2rived9 15 , 2830 .all of t hese for ms possess an integument co nsisting separate lobes surrounding t henucellus , t he lobes f used to o ne anot her towards t he chalaza wit h t he degree of f usio n varying in dif2ferent taxa . such ovules wit h an inco mplete integumentary s

37、urround are ter med“p reovules”to dis2tinguish t hese p rimitive for ms f ro m st ratigrap hically younger ovules , which posses a co mplete integu2mentary surround except for a microp ylar opening. figure 1 illust rates t he range in morp hology oft he currently identified p re2carbo niferous sper

38、matop hytes wit h four different gross arrangement s i2dentifiable .these most f requently occurring of t hese arrangement s is characterised by multiple p reovulesborn in a single cup ule ( fig.1) , including t he taxa el ki nsi a pol y m orpha rot hwell , scheckler etgillespie28, m ores net i a z

39、alesskyi ( stockmans) fairo n2demaret et scheckler12 , a rchaeosper m aa r nol dii pet tit t et beck31, xenot heca devonica ( arber et goode) emend. hilto n et edwards15 ,and ker ryi a m at tenii rot hwell et wight 30 and t wo unnamed taxa f ro m sout h wales13 . however , hilto n and edwards15 susp

40、ected t hat el ki nsi a and m ores net i a to be generically syno nymous wit ht he earlier established genus xenot heca .fig. 1 gross morp hologies and st ratigrap hical occurrences of t he earliest spermatop hytesc = cup ulate , a = acup ulate , w = winged , o = ot her . diagrams redrawn f ro m pet

41、 tit t and beck ( 1968) , fairo n2demaret and scheckler ( 1987) , rot hwell and scheckler ( 1988) , rot hwell and wight ( 1989) , hilto n and edwards( 1996 ,in p ress) and rowe ( 1992) . l n . ret is pora lepi dophy t a - v erucosis pori tes ni t i d us ; l e. ret is pora lepi dophy t a - hy m enoz

42、onot2ri letes ex pl anat us ; ll . ret is pora lepi dophy t a - knox is pori tes l i terat us ; l v . ret is pora lepi dophy t a - a picul i ret usis pora ver rucosa ;vco . ret is pora lepi dophy t a - gran dis pora corn ut a mio spo re biozo nes af ter hi ggs et al (1988) .three taxa are characteri

43、zed as individual stalked p reovules seemingly lacking a cup ule ( i . e .a glos per m a qu ad rapa rt i t a hilto n et edwards14 ,acup ulate sens u hilto n and edwards , 1996) :pseu dos porogni tes halei stockmans , 194832 and xenot heca bert ran di stockmans32 . bot h pseu2 dos porogni tes and x .

44、 bert ran di have recently been re2investigated by t he aut hor and identified as acup ulate sper matop hytes. these acup ulate for ms are f urt her distinguished f ro m t he cup ulate taxa by having integumentary lobes flat tened in cross sectio n unlike t he cup ulate taxa , which have terete in2

45、tegumentary lobes , each rounded in cross sectio n . re2investigatio n of x . bert ran di has indicated t hat t hese specimens do not belo ng to t he genus xenot heca as diagnosed hilto n and edwards15 and f urt her more , not all of t he specimens of t his taxo n as depicted by stockmans32 are inde

46、ter minate or poorly p reserved specimens of m ores net i a z alesskyi as noted by fairo n2demaret and scheckler12 .the ot her t wo gross arrangement s in late devo nian sper matop hytes are rep resented by mo no2 t ypic species. the first of t hese is dori n not heca st reel ii fairo n2demaret 10 w

47、hich has certain simi2 larities to t he acup ulate p reovules discussed above , except t hat it has a branched cup ule2like st ruct ure at tached towards t he base of t he p reovule . whet her t his is an extensio n of t he integument or if it rep resent s an additio nal layer (i . e . a cup ule) is

48、 not yet certain . the gross morp hology of t his p re2 ovule is st rikingly dissimilar f ro m all ot her currently identified late devo nian sper matop hytes al2 t hough sevral of it s individual morp hological feat ures are known in ot her devo nian sper matop hytes. the final gross morp hology ,

49、show in w a rstei ni a pap rot hii rowe29 , possesses an integument which is laterally extended to for m four small wings surrounding a hydrasper man t ype nucellus. whet her w a rstei ni a is borne in a cup ule or not is unknown as it has o nly been identified in a dis2 persed state .this morp holo

50、gical diversit y in t he earliest sper matop hytes most likely reflect s an adaptive radi211 期j aso n hilto n : 种子植物的起源及其早期演化 (英)985atio n , wit h t he sper matop hytes quickly exploiting t he selective advantages int roduced by t heir uniquerep roductive st rategy. in t his case , individual lineag

51、es show marked adaptatio n to p resumably differ2 ent enviro nmental sit uatio ns , including sper matop hytes wit h bet ter dispersal potentials (e . g. winged for ms) and bet ter p rotectio n ( e . g. cup ulate for ms) and bet ter pollinatio n potential ( see niklas33 for f ull synop sis) . the f

52、ull implicatio ns of k2 and r2selective morp hologies has not been adequately assessed alt hough winged seeds such as w a rstei ni a p resumably rep resent r2selective or pio neering plant st rategies , as opposed to t he of ten larger acup ulate for ms which p robably rep resent k2selective or esta

53、blished system plant s.wit h t he exclusio n of dori n not heca , t hese early seed architect ural models are not unique to t he devo nian period , wit h similar for ms occurring in t he fossil record , especially during t he lower car2 bo niferous. examples of lower carbo niferous seeds include win

54、ged taxa ( e . g. l y ras per m a and s a m a ropsis) , cup ulate taxa ( e . g. s t a m nostom a and gnetopsis ) and stalked ( ? acup ulate) taxa (e . g. genom osper m a) . the devo nian sper matop hytes can t herefore be co nsidered to rep resent a period of early morp hological experimentatio n f

55、ro m which several of t he important and numerically do minant subsequent seed morp hot ypes originated.d evo nia n seed2l i ke structuresfossil records f ro m t he devo nian period include so me unusual and interesting seed2like st ruc23t ures ( t he seed2analogues of rice et al 34 t hat despite lo

56、o king like ovules and seeds , have not at2)tained t he gross levels of rep roductive organisatio n seen in t he sper matop hytes. the oldest of t hese iss phi n x i a w u hani a li , hilto n et hemsley35 f ro m hubei province in cent ral china , which occur in t he lower part of t he upper devo nia

57、n ( frasnian) , several millio n years p rior to t he first occurrence of t he sper matop hytes. despite co ntaining a single enlarged f unctio nal megaspore (i . e . a mo no megasporangium) which is enclosed in a p rotective“integument2like”st ruct ure , s phi n x i a lacks t he de2velop ment of a nucellus (especially t he nucellar apex for pollen receptio n and retentio n p rior to fertili2 satio n) and indehiscence cannot be adequately p roven . the p resence of bot h indehiscence and a nu2 cellus chara