Molecules, Morphology, Fossils, and the Relationship of Angiosperms and Gnetales

15 Pages·1998·0.35 MB·English

by  Doyle, J. A.

MOLECULARPHYLOGENETICSANDEVOLUTION
Vol.9,No.3,June,pp.448–462,1998
ARTICLENO.FY980506
Molecules, Morphology, Fossils, and the Relationship
of Angiosperms and Gnetales
JamesA.Doyle
SectionofEvolutionandEcology,UniversityofCalifornia,Davis,California95616
ReceivedMay15,1997;revisedDecember16,1997
angiosperms in having special water-conducting ves-
Morphologicalanalysesofseedplantphylogenyagree sels in the wood, and they all have reproductive struc-
thatGnetalesaretheclosestlivingrelativesofangio- tures organized into compound strobili and seeds with
sperms,butsomestudiesindicatethatbothgroupsare amicropylartube.Recently,Friedman(1990,1992)and
monophyletic,whileothersindicatethatangiosperms Carmichael and Friedman (1996) confirmed earlier
are nested within Gnetales. Molecular analyses of
reports that Gnetales have part of the angiosperm
several genes agree that both groups are monophy-
process of double fertilization: both of the two sperm
letic,butdifferonwhethertheyarerelated.Conflicts
producedbythemalegametophyte(inthepollen)fuse
amongmorphologicaltreesdependontheinterpreta-
with nuclei in the female gametophyte (in the ovule),
tion of certain characters; when these are analyzed
butthesecondfusionproducesanadditionalembryoor
critically, both groups are found to be monophyletic.
embryos rather than triploid endosperm tissue, as in
Conflictsamongmoleculartreesmayreflecttherapid
angiosperms. The other living seed plants (cycads,
Paleozoicradiationofseedplantlines,aggravatedby
the long branches leading to extant taxa. Trees in Ginkgo,andconifers)haverarelybeenassociatedwith
whichangiospermsarenotrelatedtoGnetalesconflict angiosperms.
morewiththestratigraphicrecord.Evenifmolecular Moleculardatamayalsohelpclarifythe‘‘rooting’’of
dataresolvetherelationshipsamonglivingseedplant the angiosperms, or determining which angiosperm
groups, understanding of the origin of angiosperm groups are ‘‘basal’’(i.e., derived from the first splits in
organswillrequireintegrationoffossiltaxa,necessar- angiosperm phylogeny). This may help in reconstruct-
ilyusingmorphology. r1998AcademicPress ing states in the common ancestor of living angio-
spermsandthusinconnectingthemwithothergroups.
However, in this article I will consider only relation-
shipsofangiospermswithotherseedplants.
INTRODUCTION
At first sight, morphological and molecular data on
Oneofthemostintractableproblemsinplantsystem- the relationship of angiosperms and Gnetales may
atics and evolution is the origin of angiosperms, or appeartoconflict,butcloserexaminationsuggeststhat
flowering plants, the dominant plant group on earth they are actually complementary. On one question,
today.Oneaspectofthisquestioniswhichoftheother whetherangiospermsandGnetalesarebothmonophy-
seedplants,or‘‘gymnosperms,’’aremostcloselyrelated letic,moleculardataareinagreement,whereasmorpho-
to angiosperms—their closest outgroup (sister group) logical analyses have given inconsistent results. How-
oroutgroups.Toapaleobotanist,thisinvolvesaconsid- ever, on the question of whether the two groups are
eration of the many extinct Paleozoic and Mesozoic relatedatall,itisthemoleculardatathatareinconsis-
seed plants, but to a molecular systematist, who can tentandmorphologicaldatathatareunanimous.Iwill
study only extant organisms, the question is which argue that the disagreements among morphological
living seed plants are closest to the angiosperms. The analyses are due to questionable interpretations of
only living group that has attracted significant atten- characters, and when these are corrected one obtains
tion is the Gnetales (Martens, 1971; Friedman, 1996), results that agree with molecular data. The disagree-
which include three divergent genera. Gnetum is a ments among molecular analyses may reflect inherent
tropical rainforest tree or vine, with leaves virtually limitationsofmoleculardatainresolvingancientradia-
identical to those of a dicotyledonous angiosperm. tions. I will also argue that consideration of the great
Ephedra is a desert shrub with small, pointed leaves, diversityoffossilplantsisnecessaryinordertounder-
but as in Gnetum these are opposite. Welwitschia, in standtheoriginofangiospermorgans,sincestructures
thedesertsofsouthwestAfrica,producesonlyonepair in living seed plants are too highly modified, and that
ofstrap-shapedleaves.Allthreegeneraresemblemost fossils may provide independent tests of hypotheses
448
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RELATIONSHIPOFANGIOSPERMSANDGNETALES 449
basedonmoleculardata,bothasadditionaltaxaandas Others have linked angiosperms to the Permian
sources of stratigraphic evidence that bears on tree glossopterids, which had simple leaves with simple
topology. Since fossils can be integrated into phylog- reticulatevenationandleaflikeovulatestructuresbear-
enies only through their morphological characters, ing one or several cupules or ‘‘sporophylls’’ (Stebbins,
theseareargumentsforcontinuedattentiontomorphol- 1974; Retallack and Dilcher, 1981). The cupules could
ogy, even if molecules become the primary basis for be transformed into bitegmic ovules as proposed for
reconstructingrelationshipsamonglivinggroups. Caytonia, while the carpel could be derived from the
leaflikeportion.
HYPOTHESES ON RELATIONSHIPS
PhylogeneticAnalysesBasedonMorphology
PrecladisticViews
Oftheseveralcladisticanalysesofseedplantsbased
One of the earliest hypotheses, proposed by Wett- on morphological data, all agree that Gnetales are the
stein (1907) but often called the ‘‘englerian’’ theory,
closestlivingrelativesofangiosperms,butsomeanaly-
postulated that angiosperms were derived from Gne-
ses indicate that both groups are monophyletic, while
tales, which were in turn related to conifers. This was
others indicate that angiosperms are nested within
basedoncomparisonsbetweenthecompoundstrobiliof
Gnetales. They also differ in how fossils are interpo-
Gnetales, which consist of bracts with simple, uni-
lated among living taxa—some link angiosperms and
sexual ‘‘flowers’’ in their axils, with one orthotropous
GnetaleswithCaytoniaandglossopterids,whileothers
(erect) ovule in the female flowers, and the inflores-
link them with coniferopsids (conifers, ginkgos, Paleo-
cencesofthewind-pollinated‘‘Amentiferae,’’whichare
zoic cordaites). For this reason, they vary greatly in
also made up of simple, unisexual flowers, with one
implications for character evolution, such as origin of
orthotropous ovule. This view fell out of favor with
the carpel. Some of these scenarios might never be
increasing evidence that Amentiferae are advanced
envisionedifonlylivingtaxawereconsidered,asisthe
within angiosperms, compared to ‘‘Magnoliidae,’’ such
casewithmolecularanalyses(cf.DoyleandDonoghue,
as Magnolia, with showy, bisexual flowers, and this
1987,1992).
conclusionhasbeenamplyconfirmedbybothfossiland
In the analysis of Crane (1985), angiosperms and
moleculardata(Doyle,1978;Chaseetal.,1993;Crane
Gnetales were sister groups. The two were in turn
et al., 1995). However, englerian ideas are far from
related to Bennettitales and the Mesozoic genus Pen-
dead, since one magnoliid group, Chloranthaceae, is
toxylon, and the whole clade was associated with
also like Gnetales in having opposite leaves, inflores-
glossopterids, Caytonia, and Triassic corystosperms.
cencesofsimpleflowers,andoneorthotropousovule.
Since Bennettitales, Gnetales, and angiosperms all
A competing theory was proposed by Arber and
haveflowerlikestructures,thiswouldsupportanArber
Parkin(1907),inspiredbythelarge,bisexual‘‘flowers’’
and Parkin (1907) scenario for floral evolution, with
ofMesozoicBennettitales,whichhadcycad-likeleaves.
angiosperm carpels and bennettitalian stalked ovules
In Bennettitales, the ovules were borne on simple
both derived from pinnate sporophylls like those of
stalks, rather than anything like a carpel, the closed
Caytonia.
ovule-bearingorganofangiosperms.ArberandParkin
To test these results, Doyle and Donoghue (1986,
therefore proposed that the two groups were derived
1992) added several characters that might support
fromacommonancestorwithpinnatesporophylls,with
alternative relationships, but this had little effect
the megasporophylls folded to enclose the ovules in
(Fig.1).Mostimportantly,angiospermswereseparated
angiosperms, but reduced in Bennettitales.Arber and
Parkin(1908)thoughtthatGnetaleswerealsorelated from Gnetales, as the sister group of Bennettitales,
toangiosperms,buttheyinterpretedtheirsimpleflow- Pentoxylon, and Gnetales. These four groups were
ersasreducedratherthanprimitive. called ‘‘anthophytes’’ because their common ancestor
Angiosperms have also been associated with the wouldhavehadflowerlikestructures.Caytoniawasthe
Mesozoic genus Caytonia, which had palmately com- sistergroupoftheanthophytes,supportingthehomol-
poundleaveswithsimplereticulatevenationandmega- ogyofthebitegmicovulewiththeCaytoniacupule.The
sporophylls bearing two rows of anatropous (reflexed) flowersofGnetaleswouldrepresenttheculminationof
‘‘cupules,’’ each containing several ovules (Gaussen, a trend for reduction and simplification. Ovules of
1946;Stebbins,1974;Doyle,1978).Thiswouldexplain Gnetales have two integuments; the outer of these
the fact that the ovules of angiosperms are usually would correspond to the perianth of Bennettitales.
anatropousandbitegmic(withtwointeguments).Ifthe These inferences illustrate the importance of fossils:
number of ovules in a Caytonia cupule was reduced to without fossils interpolated among extant taxa, there
1, the cupule would resemble an angiosperm ovule, wouldbenoCaytoniaprototypeforthebitegmicangio-
withtheouterintegumentcorrespondingtothecupule spermovule,andtheouterintegumentofangiosperms
wall. The carpel might be derived by widening and might be derived from the outer integument of Gne-
foldingofthesporophyllrachis. tales.
450 JAMESA.DOYLE
FIG.1. RepresentativemostparsimoniousseedplanttreeofDoyleandDonoghue(1992),showingevolutionofleafmorphologyanda
scenarioforfloralevolution,withshadingindicatinghomologousparts(modifiedfromDoyle,1994).Namesofextanttaxaaregiveninboldface
type.
Unfortunately, these results were not robust: Doyle spond to the subtending bract (Doyle, 1994).
and Donoghue (1987, 1992) found very different trees AnthophyteswerealsolinkedwithconifersinRothwell
that were only one or two steps longer. These include and Serbet (1994), while angiosperms and Gnetales
‘‘neo-englerian’’ trees, with Gnetales basal in antho- were sister groups within anthophytes. Angiosperms
phytes and anthophytes linked with conifers (Fig. 2). and Gnetales were linked with conifers in analyses of
This would imply that the simple flowers of Gnetales livingplantsonlybyLoconteandStevenson(1990)and
are primitive and homologous with the fertile short DoyleandDonoghue(1992).However,thisresultwould
shoots of Paleozoic conifers. The angiosperm outer notnecessarilyimplyaneo-englerianscenarioforfloral
integument might be derived from the outer integu- evolution, since it would also be consistent with trees
ment of Gnetales, itself derived from sterile append- with glossopterids and Caytonia interpolated between
ages on the fertile short shoot; the carpel might corre- conifersandanthophytes.
RELATIONSHIPOFANGIOSPERMSANDGNETALES 451
FIG.2. Representativemostparsimoniousneo-engleriantreebasedonthedatasetofDoyleandDonoghue(1992),showingevolutionof
leafmorphologyandascenarioforfloralevolution(modifiedfromDoyle,1994).
Another weakness of these studies is that they (1994)includedninepotentiallybasalangiospermtaxa,
treated angiosperms as a single taxon, which required to which Doyle (1996) added two more and a Jurassic
debatable assumptions on basic states. For example, relative of Gnetales, Piroconites (Kirchner, 1992; van
Doyle and Donoghue (1986, 1992) scored angiosperms Konijnenburg-van Cittert, 1992), which had glossop-
as having spiral leaves and anatropous ovules. How- terid-like reproductive structures. The two analyses
ever, opposite leaves and orthotropous ovules occur in gavesimilarresults.InDoyle(1996),Gnetaleswerethe
Chloranthaceae, which are among the oldest angio- closest living group to angiosperms, but Caytonia was
sperms in the Cretaceous fossil record (Crane et al., located on the angiosperm line, rather than below the
1995) and have been considered a link between angio- anthophytes,andinmanytreesglossopteridswerethe
sperms and Gnetales (Meeuse, 1972; Hickey and Tay- sister group of anthophytes (Fig. 3). This implies that
lor,1996). thecommonancestorofangiospermsandGnetaleshad
In an attempt to correct this problem, Doyle et al. glossopterid-like ovulate structures, retained up to
FIG. 3. Representative most parsimonious seed plant tree of Doyle (1996), showing evolution of leaf morphology and a scenario for
evolutionoftheovulatestructures.GNET,Gnetales.DechellyiaisaLateTriassicfossiltooincompletelyknowntobeincludedintheanalysis.
452
RELATIONSHIPOFANGIOSPERMSANDGNETALES 453
Piroconites on the line to Gnetales, and anatropous short length of sequences and small number of taxa
cupulesaroseontheCaytonia–angiospermline,rather analyzed: Hori et al. (1985), based on 5S rRNA, which
thanbeingancestralinanthophytes. did not include Gnetales; and Troitsky et al. (1991),
In contrast, in the analysis of Nixon et al. (1994), based on 5S, 5.8S, and short 18S sequences, which
which included 18 angiosperms, angiosperms were separatedGnetumandEphedra,possiblybecauseWel-
nestedwithinGnetales,whichwerethereforeparaphy- witschiawasnotincluded.
letic rather than monophyletic (Fig. 4). Angiosperms In Hamby and Zimmer (1992), based on most of the
were linked with Gnetum and Welwitschia, whereas 18S rRNA gene and smaller portions of 26S, angio-
Ephedra was one or two nodes lower. In most trees,
spermsandGnetaleswerebothmonophyletic,buttheir
anthophytes were linked with conifers, never with
relationships were unstable. In a neighbor-joining
glossopterids and Caytonia. These results imply that
analysisof60taxa,withseedplantsrootedbyPsilotum
angiosperms were derived from a gnetalian proto-
and Equisetum, Gnetales were linked with angio-
type—a neo-englerian scenario (cf. Fig. 2). Consistent
sperms, while cycads, Ginkgo, and conifers formed a
with this, Chloranthus or Casuarina was basal in
cladeatthebaseofseedplants(Fig.5e).Inaparsimony
angiosperms,ratherthanMagnolialesorNymphaeales
analysisthetwoangiospermoutgroupswerereversed,
(Donoghue and Doyle, 1989; Doyle et al., 1994; Doyle,
sothatGnetaleswerebasalinseedplants(Fig.5f),but
1996). Angiosperms were also nested within Gnetales
this was favored over the other arrangement by only
in a smaller analysis by Hickey and Taylor (1996),
onestep.When72taxawereanalyzedwithparsimony
linkeddirectlywithGnetum.
(Doyleetal.,1994),therelationshipofangiospermsand
PhylogeneticAnalysesofMolecularData Gnetales was favored. Both trees correspond to the
In contrast, all major molecular analyses strongly same unrooted tree of seed plants; the difference is
support angiosperms and Gnetales as monophyletic whetheroutgroupsattachtothelinetocycads,Ginkgo,
groups, thus contradicting Nixon et al. (1994) and andconifersorthelinetoGnetales.Thereisreasonto
HickeyandTaylor(1996),buttheydisagreeonwhether question any rooting based on living outgroups, which
the two groups are related at all. Simplified molecular branchedfromseedplantsbeforetheMiddleDevonian.
trees are presented in Fig. 5, together with arrange- Inmorphologicalanalysesincludingfossils(Figs.1–4),
ments of extant taxa based on morphology (Figs. 5a– the crown-group (all derivatives of the most recent
5d). I will pass over two early studies because of the common ancestor of extant seed plants) was rooted
FIG.4. RepresentativeseedplanttreeofNixonetal.(1994).
454 JAMESA.DOYLE
FIG.5. Relationshipsoflivinggroupsfoundinmorphologicalandmolecularanalysesofseedplants.
with Late Devonian–Carboniferous progymnosperms only 63%. Gnetales were basal in gymnosperms, so
andseedferns,whicharecloserthanlivingoutgroups, angiosperms and Gnetales could be linked if seed
and the root was attached among cycads, Ginkgo, and plants were rooted differently. Stefanovic (1996) found
conifers. a similar tree based on 28S rDNA; again, bootstrap
Doyle et al. (1994) performed bootstrap and decay supportforthegymnospermswaslow,66%.
analyses to evaluate the strength of the rRNAresults. Incontrast,whenHasebeetal.(1992)analyzedtheir
BecausetheyquestionedtherootingbasedonPsilotum rbcLdatawithmaximumlikelihood,angiospermswere
and Equisetum, this was an unrooted analysis. Angio- the sister group of gymnosperms but Gnetales were
spermsandGnetaleswerestronglysupportedasclades, nested within the latter, linked with cycads (Fig. 5h).
at bootstrap percentages of 100 and 99%, respectively, This tree cannot be converted into one with angio-
andthetwowerelinkedatthe88%level.Thisdoesnot sperms and Gnetales linked by rerooting seed plants.
directly evaluate whether angiosperms and Gnetales Similar trees, but with Gnetales linked with conifers
form a clade, but it does measure support for an (Fig.5i),werefoundbyGoremykinetal.(1996),based
unrooted tree in which the two taxa could be a clade on chloroplast rDNA ITS sequences, by Chaw et al.
undersomerootings. (1997),basedon18SrDNA,andbyBoweandDePam-
Trees with Gnetales basal in seed plants and angio- philis (1997), based on the mitochondrial gene coxI.
sperms linked with cycads, Ginkgo, and conifers (Fig. However, in Goremykin et al., bootstrap support was
5f)werealsofoundinanalysesofthechloroplastgene only 53–58% for the gymnosperms and 50–56% for
rbcL by Albert et al. (1994) and Doyle and Sanderson Gnetalesplusconifers,incontrastto90–92%forangio-
(1997).Inthelatteranalysis,bootstrapsupportforthis sperms and 99–100% for Gnetales. In Chaw et al.
rooting (i.e., for seed plants other than Gnetales) was (1997), bootstrap support was 100% for both angio-
83%,butGnetalesandangiospermsweremonophyletic sperms and Gnetales, but only 56–75% for gymno-
atthe96and99%levels,respectively.However,Hasebe sperms.SupportforGnetalesplusconiferswashigher,
etal.(1992)foundadifferenttreewhentheyconverted 84%, but it should be noted that this is less than the
rbcL data into amino acid sequences and analyzed 88% value for angiosperms plus Gnetales in the rRNA
them by neighbor joining, with angiosperms the sister analysis of Doyle et al. (1994). Chaw et al. (1997) cited
group of other seed plants (Fig. 5g). Gymnosperms absences of angiosperm indels as evidence for the
thereforeformedaclade,butitsbootstrapsupportwas monophyly of gymnosperms, but such absences are
RELATIONSHIPOFANGIOSPERMSANDGNETALES 455
presumably symplesiomorphies that occurred in the ontheanalysisofNixonetal.;thatofHickeyandTaylor
common ancestor of all seed plants and are therefore wasbasedontoofewtaxaandcharacterdefinitionstoo
irrelevanttothestatusofgymnosperms. heavilylinkedwithoneevolutionaryscenario.
Evenwithoutconsideringindividualcharacters,there
is evidence that support for the paraphyly of Gnetales
DISCUSSION
in the Nixon et al. (1994) data set is weak. In an
AssummarizedinFig.5,allmorphologicalanalyses analysis of this data set, Donoghue, Doyle, and Fried-
(Figs.5a–5d)agreethatangiospermsandGnetalesare man(citedinDoyle,1996)foundthatbootstrapsupport
related, but some indicate that both groups are mono- for the angiosperm–Gnetum–Welwitschia clade was
phyletic (Figs. 5a–5c), while others indicate that Gne- only 54%. Furthermore, when Albert et al. (1994)
tales are paraphyletic and angiosperms are nested combined the Nixon et al. data set with rbcL data for
within them (Fig. 5d). The relationships of other seed extant taxa, they obtained trees in which Gnetales
plants are poorly resolved, but they always form a weremonophyletic.Anevenmoreintriguingresultwas
paraphyletic basal series. Molecular studies (Figs. 5e– that anthophytes were linked with Caytonia and glos-
5i) all strongly support the monophyly of both angio- sopterids rather than with conifers. This may seem
sperms and Gnetales. Some indicate that the two paradoxical, since there are no molecular data on
groups are related (Fig. 5e), but most imply that they Caytonia. However, it can be explained as a result of
are not. In some analyses (Figs. 5f and 5g), angio- strong molecular support for Gnetales, weak morpho-
sperms and Gnetales could be related if seed plants logicalsupportfortheangiosperm–Gnetum–Welwitschia
were rooted differently, but in others (Figs. 5h and 5i), clade,andthefactthatangiospermssharemorphologi-
they cannot be related under any rooting. However, calfeatureswithCaytonia,evenintheNixonetal.data
supportforthesearrangementsisweak. set.WhenangiospermsarenestedwithinGnetales,the
Inthefollowingsections,Iwilladduceevidencethat latter features have no effect on the position of antho-
themolecularresultsaremorecredibleincaseswhere phytes as a whole. But when the rbcL data pull
theirinternalsupportisstrong—thatangiospermsand Gnetales together as a clade, angiosperms go to the
Gnetales are both monophyletic—and the morphologi- baseofanthophytes,andtheirfeaturesdrawthewhole
cal results where these are strongly supported—that grouptogetherwithCaytonia.
thetwogroupsarerelated.Itshouldbenotedthatthe The key characters responsible for the result of
disagreements among analyses of the two kinds are Nixon et al. (1994) are the five synapomorphies that
somewhatdifferent.Withthemolecularstudies,many linkangiospermswithGnetumandWelwitschia,tothe
oftheconflictsarebetweenanalysesofdifferentgenes exclusion of Ephedra. Consideration of these charac-
(although some involve method of analysis or taxon ters (documented in Doyle, 1996) may illustrate some
sampling), whereas with the morphological analyses general principles about pitfalls in the use of morpho-
theconflictsarebetweendatasetsmadeupofoverlap- logical characters and ways to avoid them. These
pingcharacters,which,however,havebeeninterpreted examplesemphasizethenecessityofconsideringstruc-
differently. The fact that molecular systematics in- turesintheirpositionalanddevelopmentalcontexts.
volves fewer problems of character analysis (except Onecharacteriscellularizationofthefemalegameto-
sometimes in sequence alignment) is one of its great phyte(intheovule).Thebasicconditioninseedplants
strengths.Theconflictsamongmorphologicalanalyses isalveolar:gametophyteontogenybeginswithaphase
might thus seem to confirm the view that morphologi- of free-nuclear divisions and then cell walls form in a
cal data are hopelessly subjective and can be reinter- honeycomb-likepatternaroundindividualnuclei.How-
pretedtosupportanyhypothesisonelikes.Partofthe ever, Gnetum and Welwitschia show a nonalveolar
following discussion is therefore a defense of morphol- pattern, in which cell walls form irregularly around
ogy against this charge: it is often possible to decide several nuclei. In angiosperms, the female gameto-
objectively between interpretations of characters. In phyteisreducedtoaneggcellandtwosynergidsatthe
addition,Iwillarguethatfossilsmayprovideindepen- micropylar end, three antipodal cells at the other end,
dentcriteriaforchoosingamongmolecularhypotheses. and two free polar nuclei in the middle. Nixon et al.
(1994) scored angiosperms like Gnetum and Wel-
ConflictsamongMorphologicalAnalyses
witschia,presumablybecausethemiddleofthegameto-
With morphological data, the main disagreement on phyte remains free-nuclear. However, the egg, syner-
relations of living taxa concerns whether Gnetales are gids,andantipodalsofangiospermsarealluninucleate
monophyleticorparaphyletic,asinNixonetal.(1994) cells, and in this sense they are more like other seed
and Hickey and Taylor (1996). One could simply claim plants. One solution would be to score angiosperms as
that the molecular data refute these hypotheses, and unknown. However, it is also possible to redefine the
this might be a valid position. However, the same character in terms of a related but less ambiguous
conclusioncanbereachedbycloserexaminationofthe distinction: cellularization resulting in uninucleate vs
relevant morphological characters. I will concentrate multinucleatecells.Underthisdefinition,angiosperms
456 JAMESA.DOYLE
canbeunambiguouslyscoredashavingthebasicstate three or more sterile nuclei, the last called the stalk
and Gnetum and Welwitschia the derived state. This cell,andtwosperm,butinGnetumandWelwitschiathe
suggests a general principle: when choosing among numberofnucleiisreducedtofourandinangiosperms
alternative distinctions between character states, one tothree.However,reductionfromfivetofournucleiin
should define states such that critical taxa can be GnetumandWelwitschiaoccursbylossofthestalkcell,
scoredunambiguously. so the changes in the two characters are equivalent.
A related character is the presence or absence of Thestalkcellwouldalsobelostinangiospermsduring
archegonia, the original egg-containing organ in land reductiontothreenuclei,whetherthisoccurredbythe
plants.Ephedrahasarchegonia,withaneggandneck same or another route. Treating the stalk cell as a
cells,buttheyareabsentinangiosperms,Gnetum,and separatecharacterthereforecountsthesameevolution-
Welwitschia. This may seem like a clear, objective ary event twice; on the general principle that such
distinction, but closer examination shows that the weighting should be avoided, this character should be
conditions in angiosperms and the two gnetalian gen- eliminated.
eraareverydifferent.InGnetum,themicropylarendof When these characters are redefined in a more
the gametophyte remains free-nuclear, and free nuclei neutralmanner(Doyle,1996)andtherestoftheNixon
functionaseggs(Martens,1971;CarmichaelandFried- et al. (1994) data set is left unchanged, one obtains
man, 1996); in Welwitschia, the whole gametophyte trees in which Gnetales and angiosperms are both
becomes cellularized, but individual nuclei of multi- monophyletic. The bootstrap support for Gnetales is
nucleate cells act as eggs (Carmichael and Friedman, only moderate (75%), but this is as high as for many
personalcommunication).Incontrast,althoughangio- groups whose monophyly has rarely been questioned
spermshavenoneckcellsandthusnoarchegonia,the (e.g.,cycads).
egg is a normal, uninucleate cell. I see no basis for
ConflictsamongMolecularAnalyses
equating these two states or for considering one a
modificationoftheother;infact,onemightarguethat In discussing the results of molecular analyses, I
they resulted from different modifications of the basic noted reasons to doubt the rootings obtained for seed
gametophyte ontogeny. In the face of such divergent plantsasawhole.Seedplantsareagoodexampleofa
scenarios for character evolution, bias in favor of one radiation that occurred in a relatively short period a
scenario over another can be avoided by treating the long time ago, a situation in which molecular data are
alternativeconditions(threeinthiscase)asstatesofan leastlikelytobereliable,becauseofthesmallnumber
unorderedmultistatecharacter. ofmolecularsynapomorphiesbetweennodes,thepossi-
Similarproblemsconcernembryogeny,whichpasses bilitythatthesewereerasedbylaterchanges(multiple
throughaninitialfree-nuclearphaseincycads,Ginkgo, hits),andtherelatedproblemoflongbranchattraction
andconifersbutproceedsentirelybycellulardivisions (Felsenstein, 1978; Donoghue et al., 1989; Donoghue
inangiosperms,Gnetum,andWelwitschia.Thecritical andSanderson,1992).
taxon is Ephedra, which Nixon et al. (1994) and previ- This point is illustrated by Fig. 6, the tree in Fig. 3
ous workers (Crane, 1985; Doyle and Donoghue, 1986, (Doyle,1996)withconifersandangiospermsreducedto
1992) interpreted as free-nuclear, because the two single taxa, plotted against geologic time. Black lines
zygotesdividetoproduceeightfreenuclei.However,as show the known ranges of each group, white lines
emphasized by Friedman (1994), the following stages ‘‘ghost lineages’’ (Norell, 1992), where the cladogram
are more like those in angiosperms, Gnetum, and predictsthatthelineexistedbecauseitssistergroupis
Welwitschia, in that each of the eight free nuclei (now alreadyknowninthefossilrecord,butthelineitselfis
surrounded by cell walls) produces a separate embryo not.Ghostlineageshavebeenextendeddownwardthe
by cellular divisions, whereas in cycads, Ginkgo, and least possible distance (minimum implied gap: Benton
conifers many free nuclei contribute to one embryo. and Storrs, 1994), resulting in several apparent poly-
This suggests that the occurrence of two free-nuclear chotomies.Fourlivingseedplantlinesseparatedinthe
divisions in Ephedra is a secondary advance. Whether Carboniferous; plants on their stem-lineages occur in
or not this hypothesis is correct, it casts doubt on the theLateCarboniferousorPermian,andmostifnotall
scoring of Ephedra. Again, this character can be re- ofthemanyseedplantsknownfromtheEarlyCarbon-
placed with a related one, whether each embryo is iferous are more primitive than their reconstructed
derivedfromseveralfreenucleiorfromasinglecellby common ancestor (e.g., lacking such crown-group apo-
cellulardivisions,forwhichEphedracanbeunambigu- morphiesasendarchprimaryxylem,pollenwithhoney-
ouslyscoredashavingthelatterstate. comb-type exine structure and air sacs, and bilateral
A final case involves redundancy of two characters ovule symmetry).Although the first records of the two
used by Nixon et al. (1994): number of nuclei in the anthophyte lines are Late Triassic, it is possible that
male gametophyte (in the pollen) and presence or they diverged in the Permian. If glossopterids are
absence of a stalk cell. In the basic male gametophyte paraphyletic(theyhaveonlytwoquestionableautapo-
ontogeny in seed plants, successive divisions produce morphies, saccate and striate pollen), the ghost lin-
RELATIONSHIPOFANGIOSPERMSANDGNETALES 457
FIG. 6. Most parsimonious seed plant tree of Doyle (1996) (Fig. 3) plotted against geologic time (Palmer, 1983), approximated to
10-million-yearintervals,withknownranges,ghostlineagesimpliedbythetree,andmeasuresofconsistencywiththestratigraphicrecord
(seetextfordiscussion).RangesbasedonStewartandRothwell(1993)andothergeneralreferences.
eages leading to both lines may be represented in the (byadding10characters,withangiospermsandElkin-
Permian by plants known as glossopterids (Doyle, siascoredas0,otherlivingseedplantsas1,andfossils
1996). asunknown,thensubtracting10stepsfromtheresult-
Other insights may be gained by considering how ing trees). Contrary to Troitsky et al. (1991), this
unparsimonioustreesfoundwithmoleculardataarein arrangement does not mean that angiosperms and
termsofamorphologicaldataset(Doyle,1996).Figure gymnosperms were independently derived from Devo-
7 shows one of two trees found after forcing angio- nianprogymnosperms.Becauseofthestem,pollen,and
spermstobethesistergroupofotherlivingseedplants ovuleadvancessharedbylivingseedplants,theangio-