種子植物的起源及其早期演化

1、植 物 學(xué) 報 1998 , 40 (11) :981987act a bot a nica si nicarevie w種子植物的起源及其早期演化j aso n hilto n( 中國科學(xué)院植物研究所 北京 100093)摘要 迄今為止世界上最早的種子植物發(fā)現(xiàn)于北美和歐洲西北部的晚泥盆紀(jì)地層 。這些種子植物可能起源于前裸子植物 ,即解剖構(gòu)造上較為進(jìn)化的一類蕨類植物 。然而 ,從目前的證據(jù)來看尚無法確切判斷 這些種子植物起源于前裸子植物中的某個特定類群 ,比如古羊齒目和戟枝木目 。作者總結(jié)了種子植物 祖先的有關(guān)證據(jù) ,并對目前已知的最早的種子植物的形態(tài)進(jìn)行了概述 。由于種子植物自出現(xiàn)起即表現(xiàn)

2、出豐富的形態(tài)多樣性 ,因而尚無法對某個可能的祖先形態(tài)作出判斷 。目前的研究尚不能確切回答種子 植物起源的全部問題 ,仍需尋找有關(guān)前裸子植物與種子植物過渡類型的新證據(jù) 。作者對未來的研究提 出了建議 。關(guān)鍵詞 種子植物 , lignop hyte , 前裸子植物 , 演化 , 系統(tǒng)發(fā)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 : 種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 : 種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