Cladistics 18, 127–136 (2002)

doi:10.1006/clad.2001.0187, available online at http://www.idealibrary.com on

Molecular Rates Parallel Diversification Contrastsbetween Carnivorous Plant Sister Lineages1

Richard W. Jobson*,†,‡ and Victor A. Albert‡,2

*Department of Botany, The University of Queensland, Brisbane 4072, Queensland, Australia;†The Lewis B. and Dorothy Cullman Program for Molecular Systematics Studies, The New York Botanical Garden,Bronx, New York 10458-5126; and ‡Biodiversity and Systematics, Department of Biological Sciences,The University of Alabama, Tuscaloosa, Alabama 35487-0345

Accepted November 13, 2001

In the carnivorous plant family Lentibulariaceae, thebladderwort lineage (Utricularia and Genlisea) is sub-stantially more species-rich and morphologically diver-gent than its sister lineage, the butterworts (Pinguicula).Bladderworts have a relaxed body plan that has permittedthe evolution of terrestrial, epiphytic, and aquatic formsthat capture prey in intricately designed suction bladdersor corkscrew-shaped lobster-pot traps. In contrast, theflypaper-trapping butterworts maintain vegetative struc-tures typical of angiosperms. We found that bladderwortgenomes evolve significantly faster across seven loci (thetrnL intron, the second trnL exon, the trnL–F intergenicspacer, the rps16 intron, rbcL, coxI, and 5.8S rDNA)representing all three genomic compartments. Genera-tion time differences did not show a significant associa-tion. We relate these findings to the contested speciation

rate hypothesis, which postulates a relationship betweenincreased nucleotide substitution and increased clado-genesis. q 2002 The Willi Hennig Society

1Sequence data from this article have been deposited with the

EMBL/GenBank Data Libraries under Accession Nos AF482504–

AF482681.2To whom correspondence should be addressed at Natural His-

tory Museums and Botanical Garden, University of Oslo, Sars’ gate

1, N-0562 Oslo, Norway. E-mail: victor.albert@nhm.uio.no.

0748-3007/02 $35.00 127q 2002 by The Willi Hennig Society

All rights reserved.

INTRODUCTION

The advent of molecular phylogenetics has alerted

researchers to the issue of unequal rates of nucleotide

substitution. Lineages of organisms that exhibit long

versus short branches in phylogenetic trees have been

compared extensively for their relative rates of molecu-

lar change. In turn, explanations for rate differences

have ranged from mutational processes to whole or-

ganism effects (e.g., Bromham et al., 1996; dePamphilis

et al., 1997; Farias et al., 1999; Duret and Mouchiroud,

2000; Gissi et al., 2000; Kollman and Doolittle, 2000).

In higher plants, reproductive generation time differ-

ences, for example, in annual versus perennial plant

species, have received the most experimental support

(Gaut et al., 1997; Laroche et al., 1997). Alternatively,

the speciation rate hypothesis, which posits higher mo-

lecular evolutionary rates accompanying greater cla-

distic diversity (Barraclough and Savolainen, 2001;

Barraclough et al., 1996; Mindell, 1996), has clear prece-

dent in population biological and evolutionary theory

(Mayr, 1954; Carson and Templeton, 1984; Mindell etal., 1989; Harrison, 1991; Coyne, 1992; Orr, 1995) despite

inconclusive evidence for its role (Bousquet et al., 1992;

Muse, 2000) and mechanistic foundation (Barraclough

128

and Savolainen, 2001; Barraclough et al., 1996) in plant

molecular evolution.

Here, we demonstrate support for diversification-

generated molecular rate heterogeneity between het-

eromorphic sister lineages within the animal-trapping

plant family Lentibulariaceae (Givnish, 1989; Juniper

et al., 1989; Albert et al., 1992). Although of the same age

relative to their common ancestral node and likewise

carnivorous, the butterwort (Pinguicula) and bladder-

wort (Utricularia and Genlisea) lineages nevertheless

display striking differences in morphological and func-

tional specialization that correlate strongly with differ-

ences in rates of nucleotide substitution.

The butterworts and bladderworts were grist for

Darwin’s seminal experiments on carnivorous plant

function (Darwin, 1875). Pinguicula comprises about

46 rooted, terrestrial species that ensnare animals and

detritus on rosettes of fleshy leaves covered with muci-

lage-producing glandular trichomes (Casper, 1966;

Lloyd, 1942). Utricularia is a larger genus of ca. 214

species bearing intricate, water-filled, suction bladders

that engulf various meiofauna when trigger hairs are

stimulated (Lloyd, 1942; Taylor, 1989). Genlisea, with

about 21 species, has two-armed, subterranean micro-

fauna traps that function like lobster pots (Taylor, 1991;

Reut, 1993; Fischer et al., 2000).

We studied molecular phylogenetic relationships of

butterworts and bladderworts to better understand

their morphological and functional evolution. To mini-

mize correlation with the latter processes, two predom-

inantly noncoding plastid DNA regions comprising

four loci were sequenced and used for phylogenetic

reconstruction (trnL–F and the rps16 intron; Taberlet

et al., 1991; Oxelman et al., 1997). Two protein-coding

genes, one each from the chloroplast and mitochondrial

genomes (rbcL and coxI, respectively; Albert et al., 1992;

Cho et al., 1998), and a nuclear ribosomal gene (5.8S;

Wendel et al., 1995) were sequenced to augment molec-

ular rate comparisons.

MATERIALS AND METHODS

DNA was extracted for 69 Lentibulariaceae and 10

outgroup species using the maxi-prep method of Doyle

and Doyle (1987) or micro-prep protocol outlined in

Struwe et al. (1998). Voucher specimens are listed in

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Jobson and Albert

Table 1. Taxonomy of Lentibulariaceae species follows

Casper (1966), Fromm-Trinta (1981), Speta and Fuchs

(1982), Studnicka (1985), Zamudio (1988), and Taylor

(1989, 1991).

We amplified the group II intron contained in the

plastid rps16 gene using the rpsF and rpsR2 primers

(Oxelman et al., 1997). The trnL–F region, which con-

sists of the trnL intron (group I), the second trnL exon,

and the trnL–trnF intergenic spacer, was amplified us-

ing the tRNc and tRNf primers (Taberlet et al., 1991).

Amplification, sequencing, and alignment of partial

sequences were performed according to standard pro-

tocols (Struwe et al., 1998).

Phylogeny reconstruction was performed using the

NONA (Goloboff, 1998) and PAUP* (Swofford, 1998)

applications. Trees resulting from approximate

searches (100 random replicates with TBR branch

swapping) were further enumerated using the Parsi-

mony Ratchet method (Nixon, 1999) as implemented

in NONA. Parsimony jackknife support for internal

branches (Farris et al., 1996) was estimated using the

Xac application (J. S. Farris, unpublished) with SPR

branch swapping on each of 5 random entry orders

for 1000 jackknife replicates. The trnL–F and rps16 se-

quence data were analyzed as a combined matrix (2096

sites) since separate phylogenetic analyses of the re-

gions produced extremely compatible topologies (R.

W. Jobson and V. A. Albert, unpublished data). A strict

consensus tree was calculated from all most-parsimoni-

ous trees (24 at length 5079, consistency index 0.4920,

retention index 0.6558; Kluge and Farris, 1969; Farris,

1989) for which all branches with potential zero-length

were collapsed. Maximum and minimum internal/ex-

ternal branch lengths for the butterwort and bladder-

wort clades were calculated using a randomly selected

most-parsimonious tree with steps counted under the

ACCTRAN optimization (Farris, 1970; Swofford and

Maddison, 1987). Discussion of these phylogenetic re-

sults with reference to biogeography and character

evolution will be published elsewhere (R. W. Jobson

et al., submitted for publication).

To extend our sample of genome-wide molecular rate

differences among coding genes, we obtained partial

sequences of the plastid rbcL gene (Albert et al., 1992),

the mitochondrial coxI gene (Cho et al., 1998), and the

nuclear ribosomal 5.8S gene (Wendel et al., 1995). Mo-

lecular methods were as above except that the 1F and

724R primers (Fay et al., 1998) were used to amplify

Molecular Rate Asymmetry in Lentibulariaceae 129

TABLE 1

Accessions Sampled for Phylogenetic and Molecular Rate Analyses of Lentibulariaceae

Ingroup Voucher specimen

Genlisea aurea A. St. Hil. W. Thomas et al. 4862 (NY)

Genlisea guianensis N. E. Br. F. Rivadavia-Lopes & R. K. Sato 971A (SPF)

Genlisea violacea St. Hil. R. W. Jobson 294 (BRI)

Pinguicula agnata Casper R. W. Jobson 225 (BRI)

Pinguicula alpina Linn. F. Schuhwerk 8433/1.2 (NY)

Pinguicula caerulea Walt. H. Loconte 918 (NY)

Pinguicula elhersiae F. Speta & F. Fuchs R. W. Jobson 266 (BRI)

Pinguicula gracilis S. Zamudio R. W. Jobson 245 (BRI)

Pinguicula grandiflora Lam. R. W. Jobson 278 (BRI)

Pinguicula gypsicola T. S. Brandegee R. W. Jobson 256 (BRI)

Pinguicula lusitanica Linn. R. W. Jobson 215 (BRI)

Pinguicula moranensis H. B. & K. R. W. Jobson 257 (BRI)

Pinguicula pumila Michx. S. R. Hill 12630 (NY)

Pinguicula rotundiflora M. Studnicka R. W. Jobson 235 (BRI)

Pinguicula sp. R. W. Jobson 240 (BRI)

Utricularia adpressa Salzm. ex A. St. Hil. & Girard F. Rivadavia-Lopes & R. K. Sato 1003 (SPF)

Utricularia alpina Jacq. R. W. Jobson & D. Evans 120 (BRI)

Utricularia amethystina Salzm. ex A. St. Hil. & Girard F. Rivadavia-Lopes & M. Peixoto 871 (SPF)

Utricularia asplundii P. Taylor R. W. Jobson & J. Bockowski 125 (BRI)

Utricularia aurea Lour. R. W. Jobson 190 (BRI)

Utricularia australis R. Br. R. W. Jobson & L. Adamec 184B (BRI)

Utricularia biloba R. Br. R. W. Jobson 175 (BRI)

Utricularia bisquamata Schrank R. W. Jobson 51 (BRI)

Utricularia caerulea Linn. R. W. Jobson & T. Burson 43A (BRI)

Utricularia chrysantha R. Br. B. J. Conn 3600 (BRI)

Utricularia cornuta Michx. P. A. Fryxell 4946 (NY)

Utricularia costata P. Taylor F. Rivadavia-Lopes & R. K. Sato 1028 (SPF)

Utricularia cucullata A. St. Hil. & Girard F. Rivadavia-Lopes & R. K. Sato 941 (SPF)

Utricularia dichotoma Labill. R. W. Jobson & C. Schell 10A (BRI)

Utricularia endresii Reichb. f. R. W. Jobson 124 (BRI)

Utricularia erectiflora A. St.-Hilaire & F. Girard F. Rivadavia-Lopes & R. K. Sato 1044 (SPF)

Utricularia flaccida A. DC. R. W. Jobson 169 (BRI)

Utricularia foveolata Edgew. R. W. Jobson 115 (BRI)

Utricularia geminiloba Benj. F. Rivadavia-Lopes & J. Mullins 546 (SPF)

Utricularia geminiscapa Benj. R. W. Jobson, K. M. Cameron & T. Motley 197 (BRI)

Utricularia gibba Linn. F. Rivadavia-Lopes & R. K. Sato 974 (SPF)

Utricularia graminifolia Vahl. R. W. Jobson & M. Hochberg 106 (BRI)

Utricularia huntii P. Taylor F. Rivadavia-Lopes & R. K. Sato 1000 (SPF)

Utricularia intermedia Hayne R. W. Jobson & L. Adamec 185 (BRI)

Utricularia juncea Vahl. R. W. Jobson 136A (BRI)

Utricularia laciniata A. St. Hil. & Girard F. Rivadavia-Lopes 652 (SPF)

Utricularia lateriflora R. Br. R. W. Jobson 40 (BRI)

Utricularia livida E. Mey. R. W. Jobson 54A (BRI)

Utricularia longifolia Gardn. R. W. Jobson 76 (BRI)

Utricularia macrorhiza Le Conte J. Ricketson 4425 (NY)

Utricularia meyeri Pilg. F. Rivadavia-Lopes & R. K. Sato 1014 (SPF)

Utricularia multifida R. Br. R. W. Jobson & B. Denton 2A (BRI)

Utricularia myriocista A. St. Hil. & Girard M. Jansen-Jacobs 4717 (NY)

Utricularia nana A. St. Hil. & Girard F. Rivadavia-Lopes & M. Peixoto 883 (SPF)

Utricularia nelumbifolia Gardn. F. Rivadavia-Lopes et al. 519 (SPF)

Utricularia neottioides St. Hil. & Girard F. Rivadavia-Lopes & M. Peixoto 870 (SPF)

Utricularia nephrophylla Benj. F. Rivadavia-Lopes & J. Mullins 547 (SPF)

Utricularia olivacea Wright ex Griseb. F. Rivadavia-Lopes & R. K. Sato 973 (SPF)

Utricularia oliveriana Steyerm. R. Kral 71731 (NY)

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130

rbcL, AL27F (58-GGCTCACGCCTTTTTAATGA) and

AL28R (58-TTGCAGACAGTCCCAGCATA) for coxI,

and ITS4 and ITS5 (White et al., 1990) for 5.8S (and the

surrounding ITS regions; data not shown). Data sets

comprising four Utricularia species (U. adpressa, U. al-pina, U. geminiscapa, U. triloba), four Pinguicula species

(P. ehlersiae, P. gracilis, P. grandiflora, P. gypsicola), and

one outgroup taxon (Columnea sp., Gesneriaceae) were

constructed for the trnL–F region, rps16, rbcL, and coxI,

and were analyzed with PAUP* (Swofford, 1998) using

the exact branch-and-bound method (Hendy and

Penny, 1982). After construction of 9-taxon trees (each

resembling abstracted versions of Fig. 1 in both topol-

ogy and relative branch lengths), single, random trees

were selected, the Utricularia and Pinguicula subtrees

were pruned off sequentially, and step distributions

were calculated for each disjunct half (using the

“lscores/parsapprox” option; Swofford, 1998). Con-

servative, most-parsimonious relative rate ratio esti-

Veronica tauricola Bornm. (Scrophulariaceae)

Note. Species are separated into ingroup (Lentibulariaceae) and out

abbreviations following Holmgren et al. (1990).

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Jobson and Albert

provided by comparing mean changes per site between

the two tree halves. With the second trnL exon, no ratio

was possible since nine changes occurred in Utricularia,

whereas none occurred in Pinguicula (or the outgroup).

A data matrix of nine 5.8S sequences with identical

lineage but not species sampling was analyzed corres-

pondingly. To determine the significance of substitu-

tional differences under reasonably simple evolution-

ary process models, distance-based relative rate tests

(Robinson et al., 1998) were applied to the same data

matrices using RRTree (Robinson-Rechavi and Hu-

chon, 2000), which computes a priori group covariances

using the method of Li and Bousquet (1992). Relative

rate ratios were compared between the Utricularia and

the Pinguicula sequence subsets without the use of prior

topology information, which results in conservative

group covariance estimates. Kimura two-parameter

distances (Kimura, 1980) were used for non-protein-

mates for the Utricularia and Pinguicula lineages were coding sequences, and Jukes–Cantor distances (Jukes

TABLE 1—Continued

Ingroup Voucher specimen

Utricularia praelonga St. Hil. & Girard R. W. Jobson 73 (BRI)

Utricularia prehensilis E. Mey. R. W. Jobson & M. Hochberg 113 (BRI)

Utricularia pubescens Sm. R. W. Jobson & M. Hochberg 56A (BRI)

Utricularia purpureo-caerulea A. St. Hil. & Girard F. Rivadavia-Lopes & C. N. G. de Araujo 663 (SPF)

Utricularia reniformis A. St. Hil. F. Rivadavia-Lopes 120 (SPF)

Utricularia sandersonii Oliver R. W. Jobson & M. Hochberg 55B (BRI)

Utricularia simplex B. Br. R. W. Jobson 42 (BRI)

Utricularia simulans Pilger F. Rivadavia-Lopes & R. K. Sato 914 (SPF)

Utricularia striatula Sm. R. W. Jobson & P. Gelinaud 147B (BRI)

Utricularia subulata Linn. S. Orzell 16213 (NY)

Utricularia tricolor St. Hil. R. W. Jobson 79 (BRI)

Utricularia triloba Benj. F. Rivadavia-Lopes & R. K. Sato 1001 (SPF)

Utricularia uliginosa Vahl. R. W. Jobson & T. Burson 92 (BRI)

Utricularia violacea B. Br. R. W. Jobson 9 (BRI)

Utricularia vulgaris Linn. J. Ricketson 2897 (NY)

Outgroup

Antirrhinum multiflorum Pennell (Scrophulariaceae) R. Olmstead 846 (NY)

Byblis liniflora Salisb. (Byblidaceae) R. W. Jobson & J. Bockowski 1003 (BRI)

Ceratotheca triloba E. Mey. ex Bernh. (Pedaliaceae) R. W. Jobson 1006 (NY)

Columnea sp., Puerto Rico (Gesneriaceae) C. Lindqvist & V. Albert 30 (NY)

Digitalis grandiflora Lam. (Scrophulariaceae) R. W. Jobson 1007 (NY)

Linaria vulgaris Mill. (Scrophulariaceae) R. W. Jobson & T. Motley 1004 (NY)

Melampyrum lineare Lam. (Scrophulariaceae) R. W. Jobson 1002 (NY)

Ruellia humilis Nutt. (Acanthaceae) S. R. Hill 30665 (NY)

Uncarina grandidieri (Baill.) Stapf (Pedaliaceae) R. W. Jobson 1005 (NY)

R. W. Jobson 1008 (NY)

group blocks. Holdings in specific herbaria are so indicated with

Molecular Rate Asymmetry in Lentibulariaceae

and Cantor, 1969) were used for protein-coding se-

quences (see Robinson-Rechavi and Huchon, 2000).

Distance-based rate ratios were similar in pattern to

parsimony-based ratios, although their magnitudes

were dampened (Table 2). Finally, RRTree was used

to evaluate a possible relationship between rate and

generation time among annual and perennial species

of Utricularia and Genlisea. The test (performed exactly

as above for non-protein-coding sequences) employed

concatenated sequences from the trnL–F region and

the rps16 intron for 51 Utricularia and Genlisea species

and one Pinguicula (P. grandiflora, a relatively basal

taxon; Fig. 1) as outgroup. Five bladderwort taxa

(Utricularia chrysantha, U. endresii, U. geminiloba, U. sim-plex, U. uliginosa, and U. vulgaris) included in our phy-

logenetic reconstruction were omitted since long

stretches of missing data confounded the distance-

our trnL–F/rps16 intron sampling. An additional data

based calculations.

RESULTS AND DISCUSSION

In our phylogenetic analysis of trnL–F/rps16 intron

sequences, Pinguicula, which has the standard leaf–

shoot–root morphology typical of angiosperms, is re-

Utricularia; P, Pinguicula; K2P, Kimura 2-parameter distance; JC, Jukes–Ca K2P distances for noncoding or rDNA sequences and JC distance forb Synonymous substitution rates were not significantly different for rbc *, P , 0.05; **, P , 0.01; ***, P , 0.001.

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131

(Fig. 1). Members of this latter clade display a vast

array of leaf, shoot, and trap forms (Rutishauser and

Sattler, 1989; Taylor, 1989; Reut, 1993), while they lack

an ordinary root system (Lloyd, 1942; cf. Brugger and

Rutishauser, 1989). Among the major bladderwort

clades, trap form is highly correlated phylogenetically

(Fig. 1). Five general trap stereotypes are monophyletic,

two are paraphyletic, and one is polyphyletic (but only

twice-occurring; Fig. 1). A remarkable anagenetic pat-

tern is that maximum internal and external branch

lengths are 103 and 121 steps, respectively, in the blad-

derwort lineage, whereas corresponding branch max-

ima are 18 and 34 steps in the butterwort lineage. In

turn, this branch length difference must reflect un-

equal, underlying nucleotide substitution rates be-

tween the two clades.

We evaluated the extremity and significance of the

bladderwort/butterwort molecular rate inequality us-

ing identical, randomly selected taxon subsamples of

the aforementioned plastid loci as well as sequences

from two protein-coding genes, plastid rbcL and mito-

chondrial coxI. The proportion of butterworts to blad-

derworts sampled was balanced to counteract lineage-

size effects that may mirror sampling asymmetries in

set of 5.8S nuclear ribosomal gene sequences was exam-

solved as the sister lineage of Genlisea plus Utricularia ined, and although complete taxon overlap was not

TABLE 2

Utricularia Evolves Significantly Faster Than Pinguicula across Seven Coding and Noncoding Loci

U/P rate U/P rate

Genome, Sequence length ratio, ratio, Significance,

Locus coding status (incl. gaps) parsimony distancea distance (P) c

trnL intron Plastid, NC 510 5.600 1.471 0.004875**

trnL second exon Plastid, RC 63 — — 0.049781*

trnL–F spacer Plastid, NC 412 3.895 1.303 0.04523*

rps16 intron Plastid, NC 849 4.458 1.472 6.59 3 1026***

rbcL Plastid, PC 564 8.000 1.562 0.03724* (Ka)b

coxI Mitochondrial, PC 518 13.996 6.293 0.00809** (Ka)b

5.8S Nuclear, RC 173 3.917 1.957 0.00144**

Note. Relative rate ratios of mean changes per site were derived from parsimony step distributions on identical 4-taxon subtrees (except

5.8S; see Materials and Methods); corresponding distance-based relative rate ratios and their significance levels are reported. No rate ratio is

given for the second exon of trnL because 0 changes were observed in the P subtree; the parsimony change difference, nonetheless, is 9 steps

in the U subtree. Cellular compartment and coding status are indicated. NC, noncoding; RC, ribosomal-coding; PC, protein-coding; U,

antor distance; Ka, nonsynonymous rate.

protein-coding sequences.

cL or coxI.

132 Jobson and Albert

FIG. 1. Sequences of the plastid trnL–F region and rps16 intron reveal phylogenetic relationships and branch-length heterogeneity among

the butterworts and bladderworts. The strict consensus of most-parsimonious trees is shown. Branch lengths are proportional to the number

of nucleotide substitutions predicted by the parsimony method (scale bar 5 20 substitutions). Support values .50% from parsimony jackknife

sensitivity analysis are reported for each internal branch. Branch lengths are unevenly distributed between the strongly supported Pinguiculaand Utricularia/Genlisea clades, those of the latter being substantially longer and correlated with significant substitution rate differences among

all four loci of this analysis as well as plastid rbcL, mitochondrial coxI, and nuclear 5.8S (Table 1). Schematic representations of generalized

trap forms define groups that are principally though not totally monophyletic (Lloyd, 1942; Taylor, 1989). (A) The gland-covered leaf rosette

of Pinguicula. (B) The bifurcating, twisted traps of Genlisea, which operate like lobster-pots. (C) Suction bladders of Utricularia sections

Polypompholyx and Pleiochasia may have dorsal and ventral appendages, wings, and ventral lamellae. (D) Bladders of Utricularia section Aranellashare the dorsal and ventral appendages, the latter of which are apically bifid. (E) Bladders of five sections of Utricularia are generally

characterized by a single, protruding dorsal appendage. (F) Bladders of three sections of Utricularia are characterized by naked traps with a

slight dorsal bulge. This trap form defines a paraphyletic group. As studied by Kondo (1972), the partly sympatric and nearly identical species

Utricularia cornuta and U. juncea are reproductively incompatible. (G) Bladders of five sections of Utricularia bear two dorsal appendages. This

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aquatic. In turn, random drift acting on founder popu-

Molecular Rate Asymmetry in Lentibulariaceae

accomplished, the clade sampling proportions were

identical. Relative interclade rates for the different loci,

conservatively estimated via their parsimony step dis-

tributions, show that nucleotide substitutions in the

bladderwort lineage have accrued at a rate ca. 4 to 14

times greater than that of the butterwort clade (Table

2). Distance-based relative rate tests (Robinson-

Rachavi and Huchon, 2000) established that the blad-

derworts evolve significantly faster at all seven loci

(Table 2). For rbcL and coxI, only nonsynonymous sub-

stitution rates (Ka) were significantly different (Table

1), possibly indicating protein evolutionary disparities

between Utricularia and Pinguicula. Given that all three

genomic compartments were sampled for DNA se-

quence variation, we hypothesize that the observed

rate inequality between butterworts and bladderworts

is whole-genomic in scope.

Neither the common carnivory of butterworts and

bladderworts nor their equivalent age as sister groups

is congruent with the molecular rate patterns observed.

Likewise, there is no significant relationship between

molecular rate and generation time (P 5 0.556) in the

bladderwort lineage, which includes both annual and

perennial species (all butterworts are perennial;

Casper, 1966; Taylor, 1989). A possible factor in the

evolution of the plastid genes could be functional de-

generacy due to partial heterotrophy, but evidence for

carbon absorption by Utricularia is scant (Juniper et al.,1989), and even holoparasites in closely related Oro-

banchaceae (the broomrape family; Olmstead et al.,2001) do not uniformly show molecular rate increases

relative to hemiparasitic and fully photosynthetic taxa

(dePamphilis et al., 1997). Instead, two fundamental

evolutionary distinctions between butterworts and

bladderworts show strong correlation: (i) loss of the

typical angiosperm body plan in Utricularia and Genli-sea and (ii) cladistically correlated divergence to new,

microfaunal/meiofaunal trap forms in these taxa.

One hypothesis for the former disparity is that single

or few regulatory genetic changes could have been

a forerunner to the altered embryogeny and diverse

antennae that differ in thickness and placement at the edge of the bladd

single-appendaged, terminally borne bladders are typical of Utricularia s

respresented on our phylogenetic tree. Family membership is provided

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133

(Lloyd, 1942; Rutishauser and Sattler, 1989; Taylor,

1989). Comparable to what is seen in Utricularia germi-

nation (Lloyd, 1942), single-leaved acaulescent species

of Streptocarpus (of the closely related family Gesneria-

ceae; Moller and Cronk, 2001) have asymmetric organ-

ogenesis at the embryonic shoot apical meristem and

aborted primary root development. Furthermore, these

Streptocarpus species may be phenotypically “rescued”

to caulescence when plants are treated with exogenous

gibberellin or with auxin or arabinogalactan inhibitors

(Rosenblum and Basile, 1984; Rauh and Basile, 2000).

A comparison to the ontogeny of unifoliate species of

Streptocarpus has consequences for interpreting Utricu-laria morphology. Bladderwort traps, which can oc-

cupy an entire leaf position in embryonic shoot devel-

opment (Lloyd, 1942), might be modified leaves or leaf

pinnae (Juniper et al., 1989). Similarly, the stolons or

“rhizoids” that also cut off at the foliar position may be

modified runner-like or haustorial leaves, respectively.

As such, the trap-covered stolons of some aquatic spe-

cies may be single, multiply pinnate leaves, in keeping

with the anatomical similarity between different node–

internode levels observed by Rutishauser and Sattler

(1989). Traps in other species can also arise from

rhizoids, runner-like stolons, at leaf tips, on leaf edges,

from inflorescence floats, or from leaf lamina sur-

faces (Taylor, 1989) which further suggests that most

of the plant body is leaf. Although the early embryonic

Utricularia/Streptocarpus similarity could be homolo-

gous, it is just as likely homoplasious, perhaps involv-

ing different genes in the same signal transduction

pathway.

Regardless of the causal mechanism, the evolution

of flexible vegetative development could have been a

key innovation (e.g., Sanderson and Donoghue, 1994;

Givnish, 1997; Hodges, 1997) that reduced selection

pressure on bladderworts and allowed them to invade

underutilized, nutrient-poor niches ranging from

water-saturated terrestrial to epiphytic and suspended

lations may have promoted both divergent evolution

(cladogenesis) and higher net substitution rates (Mayr,vegetative morphology seen in the bladderwort clade

trap form occurs twice in the tree, polyphyletically. (H) Bladders of four sections of Utricularia are characterized by two branched, dorsal

er mouth. This trap form marks a paraphyletic group. (I) Naked or

ection Vesiculina. Numbers of Utricularia sections mentioned are as

for outgroup taxa.

(P. Gresshoff, J. Playford, and the Graduate School), The Lewis B.

and Dorothy Cullman Foundation, and The College of Arts and

134

1954; Carson and Templeton, 1984; Mindell et al., 1989;

Mindell, 1996) in bladderworts than in butterworts,

which were presumably more restricted from novel

habitats by their standard vegetative morphologies

(Casper, 1966).

Disruptive selection (Bates Smith, 1993; Burton and

Husband, 1999; Kondrashov and Kondrashov, 1999)

on trap form could be linked with prey specificities

observed for several species of the Utricularia/Genliseaclade (Barthlott et al., 1998; Harms, 1999; Jobson et al.,2000; Jobson and Morris, 2001). Form and function may

be tightly coupled in bladderwort traps, and as such,

major groups within the bladderwort lineage are de-

fined as well by distinct trap morphologies as they

are by molecular sequence data (Fig. 1). Additionally,

reproductive isolation is nearly complete among Utric-ularia species (Taylor, 1989); even sympatric sister spe-

cies with the same trap form and chromosome number

(U. cornuta and U. juncea) cannot hybridize (Kondo,

1972; Fig. 1). If prey fidelity and sexual incompatibility

were linked early in the foundation of the major blad-

derwort lineages, these factors could have spurred

cladogenesis and further driven the elevation of molec-

ular rates.

However, the implications of the speciation rate hy-

pothesis to bladderwort evolution may reach further.

Diversification and molecular rate increases may

become cyclical; initial substitution rates amplified

through cladogenesis could further augment the clado-

genetic rate by providing more extensive variation

among lineages to fix or eliminate (Orr, 1995). Such

cycles of cladogenetic and molecular rate increases,

initially derived from baseline variation such as that

existing in the carnivorous ancestor of butterworts and

bladderworts, could theoretically lead to bursts of ge-

netic and phenotypic diversification. In the case of

bladderworts, this process could be described as one

of punctuational evolution of new body plans (El-

dredge and Gould, 1972), limited in its scope by novel

niche availability. If indeed narrow genetic modifica-

tion initiated this process in bladderworts, other organ-

ismic radiations (e.g., Givnish et al., 1995; Baldwin,

1997) could similarly stem from few changes in genes

of large effect (Doebley and Lukens, 1998; Schemske

and Bradshaw, 1999).

q 2002 by The Willi Hennig Society

All rights reserved.

Jobson and Albert

ACKNOWLEDGMENTS

We thank C. Lindqvist, D. Oppenheimer, E. Roden, K. Roe, A.

Vahatalo, VAA’s 2001 Evolution class at The University of Alabama,

and three anonymous referees for critically reviewing the manu-

script; L. Adamec, J. Bockowski, B. Denton, D. Evans, P. Gelinaud,

M. Hochberg, F. Rivadavia-Lopez, C. Schell, P. Temple, and BRI and

NY herbaria for providing plant material; L. Struwe for laboratory

training, and K. M. Cameron for supplying sequence information.

J. S. Farris is acknowledged for permission to use the Xac parsimony

jackknifing application. Support from The University of Queensland

Sciences of The University of Alabama is gratefully acknowledged.

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