COBISS: 1.01
PATTERN AND PROCESS: EVOLUTION OF TROGLOMORPHy
IN THE CAVE-PLANTHOPPERS OF AUSTRALIA AND HAwAI’I
– PRELIMINARy OBSERVATIONS (INSECTA: HEMIPTERA:
FULGOROMORPHA: CIxIIDAE)
VZOREC IN PROCES: EVOLUCIJA TROGLOMORFNOSTI
PRI JAMSKIH MREŽEKRILNIH ŠKRŽATKIH IZ AVSTRALIJE
IN HAVAJEV – PRELIMINARNE UGOTOVITVE (INSECTA:
HEMIPTERA: FULGOROMORPHA: CIxIIDAE)
Andreas wESSEL1, Petra ERBE1,2 & Hannelore HOCH1
Abstract                                          UDC 591.542(94+739,9)
Andreas Wessel, Petra Erbe & Hannelore Hoch: Pattern and process: Evolution of troglomorphy in the cave-planthoppers of Australia and Hawai’i ? Preliminary observations (Insecta: Hemiptera: Fulgoromorpha: Cixiidae)
Te evolution of troglobites comprises three distinct problems: cave colonization by an epigean ancestor, the evolution of tro-glomorphies, and intra-cave speciation. Te study of cave-dwell-ing planthoppers has contributed much to our understanding of troglobite evolution and provides useful model systems to test various aspects of the theoretic framework developed in recent years. Most promising in this respect are taxa with several closely related but independently evolved troglobiontic lineag-es, such as on the Canary Islands, in queensland/Australia and on the Hawaiian Archipelago. Closely related species ofen oc-cur in caves with comparable ecological parameters yet difer in their age. Here we use comparative age estimates for Australian and Hawaiian cave cixiids to assess the dynamics of reductive evolutionary trends (evolution of troglomorphy) in these taxa and cave planthoppers in general. we show that the degree of troglomorphy is not correlated with the age of cave lineages. Morphological alteration may not be used to draw conclusions about the phylogenetic age of cave organisms, and hypotheses based on such assumptions should be tested in light of these fndings.
Key words: adaptive shif, cave adaptation, climatic relict, founder efect, reductive evolutionary trends, troglobites, tro-glomorphies.
Izvleček                                           UDK 591.542(94+739,9)
Andreas Wessel, Petra Erbe & Hannelore Hoch: Vzorec in proces: Evolucija troglomorfnosti pri jamskih mrežekrilnih škržatkih iz Avstralije in Havajev Preliminarne ugotovitve (Insecta: Hemiptera: Fulgoromorpha: Cixiidae)
Evolucija troglobiontov zajema tri značilne korake: kolonizacija jame s površinskim prednikom, razvoj troglomorfnosti ter podzemeljska speciacija. Študija podzemeljskih mrežekrilnih škržatkov je prispevala veliko k našemu razumevanju evolucije troglobiontov in hkrati predstavlja uporaben modelni sistem za testiranje različnih teoretičnih pristopov, ki so bili razviti v zadnjih letih. V tem pogledu so najobetavnejši tisti taksoni, ki so si sicer sorodni, toda pripadajo evolucijsko neodvisnim troglobiontskimi linijami, kot so npr. tisti na Kanarskih otokih, v državi queensland (Avstralija) in na havajskem arhipelagu. Bližje sorodne vrste se v jamah pogosto pojavijo v primerljivih ekoloških pogojih, vendar se razlikujejo v starosti. Za ugotavljanje dinamike trendov redukcijske evolucije (evolucija tro-glomorfzmov) teh taksonov in jamskih škržatkov na splošno, smo v prispevku uporabili ocene primerjalnih starosti za avstralske in havajske jamske škržatke. Ugotavljamo, da stopnja troglomorfnosti ni v korelaciji s starostjo jamskih linij. Zgolj morfološke spremembe pri organizmih se ne bi smele uporabljati za prikazovanje flogenetske starosti jamskih organizmov. Hipoteze, ki temeljijo na takšnih predpostavkah, bi morale biti preverejene v luči pričujočih ugotovitev.
Ključne besede: prilagoditveni premik, prilagoditve na podzemlje, klimatski relikt, učinek osnovatelja, redukcijski evolucijski trendi, troglobiti, troglomorfzmi.
1 Museum für Naturkunde der Humboldt-Universität zu Berlin, Biosystematics Research Group, Invalidenstrasse 43, D-10115 Berlin, Germany; e-mail: andreas.wessel@museum.hu-berlin.de
2 Chiang Mai University, Te Uplands Program, Faculty of Agriculture, Chiang Mai, Tailand
Received/Prejeto: 30.01.2007
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ANDREAS wESSEL, PETRA ERBE & HANNELORE HOCH
INTRODUCTION
Te origin of troglobites has fascinated evolutionary biologists since Darwin remarked on their curious and strong modifcation (1859: 177-178). He did not provide a ‘Darwinistic’ explanation for their evolution1, however, this was subsequently supplied by August weismann (1886)2.
Troglobite evolution, i.e. the process leading to diferent, sometimes closely related species, which are highly adapted to life in subterranean spaces, comprises three somewhat independent phenomena and problems: (i) initial cladogenesis of a cave species, or the origin of a cave-dwelling bio-species from an epigean ancestor, which is basically the problem of isolation or rather (spa-tial) separation of a cave population from its epigean relatives; (ii) subsequent anagenetic transformation, which comprises the dynamics and driving forces of cave adaptation, the ofen so-called regressive evolution or reduc-tive evolutionary trend, and, in some cases, (iii) subter-ranean (intra-cave) radiation.
RELICTS OR ExPLORERS?
A widely accepted concept aiming to explain the specia-tion event giving rise to a cave-dwelling and reproduc-tively isolated bio-species, was developed by Tomas Barr in the 1960s, commonly known as the Climatic Rel-ict Hypothesis (CRH):
1 “A s it is difcult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.” (Darwin 1859: 177)
2 „As soon as such a cave immigrant has developed the abil-ity to obtain food without the help of eyes a reduction of the eyes must commence, since as soon as the same are no longer neces-sary for the animals’ existence, they are not infuenced anymore by natural selection, because now it does not matter whether the eyes are a little worse or a little better. Now, no more selec-tion will take place between individuals with better and those with worse eyes, but both will have an equal chance to be preserved and reproduce. Individuals with better and those with worse eyes will cross from now on, and the result can only be a general degradation of the eyes. Pos-sibly this is helped by the circumstance that smaller and stunted eyes can even present an ad-vantage, since this allows other organs such as sensory and olfactory organs, which are more important for the animal now, to develop more strongly. Even without such efect, though, the lack of natural selection main-taining the eye’s high level of organization will necessarily lead to its degradation, slowly or even very slowly, especially at the beginning of this process, but in-exorably.” (Translated from the German; weismann 1886: 16-17)
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“Troglobites have evolved from colonies of troglo-philes which became isolated in caves through extinction of surface populations of the troglophiles” (Barr 1968: 96).
According to Barr, the evolution of troglobites is a two-step process: at frst, it involves a preliminary, tro-glophilic stage without apparent troglomorphies or a dis-ruption of genefow between cave-dwelling and epigean populations. Following this initial cave colonization, the cave-dwellers become geographically separated, and thus genetically isolated, due to the extinction of parental epi-gean populations (supposedly caused by climatic change), at least in the region of the cave. Over time, reproductive isolation will inevitably follow as a side efect of genetic change by drif and natural selection (Barr 1968). Support and evidence for this concept was gained from the observed relict distribution of most troglobites known at that time, which were almost exclusively confned to temperate regions. Glaciation during the ice ages was suggested as the most important factor for the change of surface conditions (Barr 1968, Sbordoni 1982, Barr & Holsinger 1985).
Tis hypothesis remained without alternatives un-til the early 1970s, when Francis G. Howarth discovered the Hawaiian cave ecosystems (Howarth 1972). Te lava tubes host, among other taxa, highly troglomorphic plan-thoppers that are parapatrically distributed with respect to their close epigean relatives, which are still extant, i.e. they are non-relictual troglobites. Consequently, How-arth (1981, 1986, 1987) formulated the Adaptive Shif Hypothesis (ASH):
“[...] potential food resource provides the driving force for the [...] evolution of cave species. Troglomor-phic populations [...] evolve from pre-adapted habitual accidentals which [...] establish temporary populations in marginal underground habitats. Once an adaptive shif occurs, allowing a reproducing population to establish itself underground, then it is both the efects of strong new selection pressures and the release from previously strong selection pressures that bring about [...] troglo-morphy” (Howarth 1986: 155).
while the exploitation of a large new habitat with new food resources may be the driving force in the evolution of troglobites according to the ASH, a major chal-lenge for survival underground is probably the ability to locate mates and reproduce in the dark. A change in mat-ing behaviour might thus have been the most important adaptive shif necessary for a successful colonization of caves, and would almost inevitably lead to reproductive isolation of the incipient cavernicolous species. Te Ha-waiian cave planthoppers provide a striking example for
PATTERN AND PROCESS: EVOLUTION OF TROGLOMORPHy IN THE CAVE-PLANTHOPPERS OF AUSTRALIA AND HAwAI’I
this process, and consequently played a pivotal role in the formation of the Adaptive Shif Hypothesis (Howarth 1986, Howarth & Hoch 2005).
A principal acceptance of the ASH does not neces-sarily invalidate the CRH, especially not in cases where the preconditions for the CRH are met, i.e. cave taxa dis-playing a relict distribution. However, a relict distribution observed today is not sufcient evidence to uncondition-ally accept the CRH, given the alternative present in the ASH 3. Te predictions arising from both hypotheses must be tested for every single system. For the CRH, we expect the closest epigean relatives at least to be allopatrically dis-tributed compared to the cave species, while the ASH pre-dicts a parapatric distribution of cave and epigean species, which are necessarily sistergroups (adelphotaxa). Conclu-sive evidence for a decision between both hypotheses may be gained from a well-founded phylogeny in conjunction with a sound knowledge of the geographic distribution of both cave and epigean taxa. Te last requirement is ofen problematic, though, as the sampling of epigean relatives for some cave species is frequently insufcient. For some groups no epigean relatives are known at all, and it is only through intensive, directed search eforts that this obstacle may be overcome (see e.g. Stone 2004).
REDUCTIVE EVOLUTIONARy TRENDS
Once a population has shifed towards a permanently cav-ernicolous mode of living, the second problem of troglo-bite evolution – subsequent anagenetic transformation – arises. A basic assumption since weismann (1886) has been a correlation between the degree of troglomorphy of a taxon and its residence time in caves. Cave adaptation is accordingly described as an orthogenetic, time-depend-ent process, which is an overall slow, gradual adaptation towards a stage of ‘absolute troglomorphy’; see e.g. wilkens (1986), for review see Barr (1968) and Howarth
“Te evidence suggests that troglobites evolve from pre-adapted habitual visitors or accidentals in the cave rather than from well-adapted troglophiles. Te former group requires an adaptive shif in order to fully exploit the cave resources. Tis adaptive shif may lead to the evolution of a troglobitic lifestyle. well-adapted troglophiles on the other hand tend to remain opportunistic exploiters of the cave environment. Some temperate troglobites may ft the scenario of isolation by changing climates (Barr, 1968). However, many species in-cluding those in the tropics probably do not. I postulate that adaptive shifs led to the colonization of caves and evolution of troglobites, including most of those in temperate caves, but that the complex geological history of the continents includ-ing glaciations has obscured the early history and obfuscated the earlier distribution and the evolution of troglobites there.” (Howarth 1981: 540)
(1987). Traditional explanations for the mechanisms of this process includes (i) the accumulation of neutral mu-tations, (ii) pleiotropic efects, and (iii) natural selection for energy economy (Sket 1986, Culver 1982). Both the CRH (Barr 1968) and ASH (Howarth 1986), however, contain some notion of a founder efect: Barr with an explicit quotation of Mayr’s genetic revolution (Mayr 1954) and Howarth with reference to the Carson model of founder efects (Carson 1968, 1975).
Te process of cave adaptation is infuenced by sev-eral parameters – such as availability of food, population density, microclimate of the caves and other biotic and abiotic factors of the cave ecosystem – , which make comparisons even between closely-related species ex-ceedingly difcult, and generalisations even more so. An excellent opportunity to test the assumption of gradual and increased troglomorphy over time may nevertheless be found in radiations of cave-dwelling planthoppers in-habiting caves of diferent age.
CAVE-ADAPTATION IN PLANTHOPPERS
Studies during the last three decades have revealed nu-merous cases of evolution of cave-adapted planthoppers in tropical and subtropical caves. Among the Fulgoro-morpha, 53 cave-dwelling species have been described from many parts of the world, four-ffh of them cixiids including the Australian taxa Solonaima and Undarana and the Hawai’ian Oliarus species (Hoch 1994, Hoch & wessel 2006).
Te adaptation to similar environments in cave planthoppers has led to the evolution of a very similar external morphology in diferent parts of the world and represents a striking example of parallel evolution. Te morphological modifcations of cave planthoppers are characterized by reductive evolutionary trends, as in most obligately cavernicolous animals. Te degree of adaptation to a subterranean life varies greatly, primarily de-pending on their habitat in the cave or soil (Fig. 1). Most conspicuous are the reduction and loss of compound eyes and ocelli, tegmina, wings and bodily pigment. It has also been suggested that apparently non-troglomor-phic characters have an increased adaptive value in the underground environment, such as e.g. the specialized spine confgurations of hind tibiae and tarsi, which may possibly enhance walking on wet or rocky surfaces (Hoch & Howarth 1989a, 1989b, Hoch 2002).
Te closest epigean relatives of cavernicolous Fulgo-romorpha species all have immature stages living close to the soil, e.g. under the dead bark of rotting logs, in leaf litter or moss, or even within the soil, feeding on roots or perhaps on fungi (Remane & Hoch 1988). Tis mode of life has been considered an ecological pre-adaptation
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Habitat	Abiotic factors	Life cycle	Morphology
epigeic (surface) habitats	unstable physical parameters	entirely epigeic / epigean	no troglomorphies
(including leaf litter)	(ambient climatic influence high)	troglophilic (facultative soil & cave-dwellers, may live & reproduce underground as well as in surface domain)	troglomorphies
hypogeic (subterranean) habitats	T stable / constant physical parameters (ambient climatic influence low)		of varying degrees e.g.
.''*¦ ^     ^- - /'* soil-    ^    . [ interstitial [                 • chambers   ) ;       meso-cavernous rock stratum     >		endogeic / endogean (obligatory soil dwellers)	- reduction of eyes, body pigmentation & wings - decreasing cuticle sclerotization
l                            caves                         ! (deep cave zone)		troglobitic (obligatory cavernicoles, restricted to cave environment)	- specialized sensory organs - elongate appendages
Fig. 1: terminology of interdependence between physical parameters of the habitat and organismic adaptations (From hoch et al. 2006).
to a later switch to a permanent (adult) life underground (Hoch 2002, Howarth & Hoch 2005).
CAVE-PLANTHOPPERS OF AUSTRALIA AND HAwAI’I
In Australia, closely related Solonaima and Undarana species have colonized old karst caves as well as younger lava tube systems. Te four epigean Undarana species oc-cur in the (rain)forest at the south of queensland’s east coast, while the two cave-dwelling species (U. rosella, Bayliss & Pinwills cave, Undara lava tube; U. collina, Col-lins cave) inhabit the lava caves of the McBride Formation in the dry grasslands westward of the Great Dividing Range (Hoch & Howarth 1989a). Te epigean Solonaima species can be found all along the east coast (rain)forest, while the cave species inhabit lava tubes within the Mc-Bride Formation, too (S. baylissa, sympatric with U. rosella), as well as karst caves of the Chillagoe Karst Towers (S. pholetor, S. stonei, S. halos, S. irvini) and Mount Mul-grave (S. sullivani) (Hoch & Howarth 1989b). Tus, epi-gean and cavernicolous species of both Australian genera show an allopatric distribution.
On the Hawaiian islands the cave-dwelling species of the endemic, monophyletic Oliarus clade represent in-dependent cave colonizations on islands of diferent age. with about 80 described epigean taxa (species and sub-species), Oliarus is the most speciose planthopper genus on the Hawaiian islands (Zimmerman 1948, Asche 1997).
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Based on morphological data, this diversity has been hy-pothesized to stem from a single colonization event (Asche 1997, Hoch & Howarth 1993). Te frst cave-dwelling species of the genus, Oliarus polyphemus Fennah, 1973 and Oliarus priola Fennah, 1973 (Fennah 1973) were dis-covered by Howarth (1972) on Hawai’i Island and Maui where they are endemic. Later, fve more troglobitic taxa were discovered on the archipelago (Hoch & Howarth 1999). Te seven cave-dwelling species owe their origin to several independent colonization events on three islands; on Molokai, one adaptive shif (O. kalaupapae); on Maui, three adaptive shifs (O. priola, O. gagnei, O. waikau); on Hawai’i Island, at least three adaptive shifs (O. polyphe-mus, O. lorettae, O. makaiki) (Hoch & Howarth 1999). Te closely related epigean species of all cavernicolous Oliarus taxa occur parapatrically at the surface.
Both the Australian and the Hawaiian cave species complexes exhibit diferent degrees of troglomorphy. Figure 2 shows the heads of six Australian Solonaima, one epigean (1), three facultative cavernicolous (2-4), and two obligate cavernicolous species (5,6). Figure 3 depicts the habitus of six Hawai’ian Oliarus species, one epigean relative on the lef (note the diferent scale), and fve troglobitic species. Te varying degree of eye reduc-tion is clearly visible; two of the Hawaiian species even show a complete loss of eyes. Te same pattern is seen in wing reduction.
Te time factor is crucial for assessing the dynamics of troglobite evolution. Unfortunately, though, it is
PATTERN AND PROCESS: EVOLUTION OF TROGLOMORPHy IN THE CAVE-PLANTHOPPERS OF AUSTRALIA AND HAwAI’I
Fig. 2: Australian Solonaima species, heads, dorsal view. 1, S. solonaima (epigean); 2. S. sullivani; 3, S. pholetor; 4, S. stonei; 5, S. irvini; 6, S. baylissa. (From hoch & howarth 1989b).
rarely possible to obtain direct estimates of the age of the cave lineages. Rather, the maximum age of the habitat is usually employed - at least if an active colonization of caves sensu Howarth is assumed - , or even just the maximum age of the underlying geological structure. By these measures, the maximum age for the troglobitic Oliarus lineages on Hawai’i is the age of the islands: 1.8 myr for Molokai, 1.3 myr for Maui, and less than 400,000 y for Hawai’i Island.
In the case of the Australian troglobitic cave plan-thoppers, the situation is even more complex. At frst sight their distribution fts the Climatic Relict Model sensu Barr very well assuming a late Miocene desertifca-tion, i.e. replacement of the rain forest by dry savannah or grassland east of the Great Dividing Range (see Kemp 1978, Truswell 1990). while not per se refuting the relict hypothesis, we do not exclude the possibility of adaptive shifs for the Australian cave planthoppers as well. In that case, Australian cave taxa may be much older than hith-erto assumed. Also, the late Miocene climatic change is not necessarily be regarded as the sole reference point for the calculation of the maximum age of the Austra-lian cave planthoppers. what could matter instead is the availability of the caves as a suitable novel habitat.
Te limestone of the Chillagoe Tower Karst and Mitchell-Palmer Karst are presumably of Silurian origin, and the current main caves were formed by phreatic so-lution during the last 5-10 million years (Ford 1978, Jen-nings 1982, Pearson 1982). Remnant older passages and solution breccias near the tops of many towers indicate the existence of caves since the area was uplifed and the limestone was exposed in the mid-Tertiary about 20-25 million years ago (Howarth 1988).
Te much younger Undara lava fow (190,000 years old) covers portions of older fows within the McBride Formation (Atkinson et al., 1976), some of which may date back from the Pliocene, i.e. more than 2.5 million years ago (Best 1983). Te cave animals could have migrated through the mesocavernous systeme into young basalt and colonized new caves in each fow in succession.
“Te troglobitic species could be, and probably are, older than the age of their caves” (Howarth 1988).
PHyLOGENETIC AGE AND TROGLOMORPHy Against this background we here attempt to assess the problem of    the dynamics of reductive evolutionary
Fig. 3: hawai’ian Oliarus species, habitus, dorsal view. 1, epigean Oliarus species (O. tamehameha); 2, O. kalaupapae; 3, O. lorettae; 4, O. gagnei; 5, O. waikau; 6. O. polyphemus. (1, from zimmermann 1948; 2-5, from hoch & howarth 1999; 6, hoch, Original).
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Fig. 4: variability of relative eye width and relative tegmina length in epigean and cave-dwelling species (O. polyphemus, O. lorettae, O. makaiki (no wing measurements), O. gagnei, O. waikau, O. kalaupapae; S. pholetor (stonei), S. irvini (halos), S. baylissa; U. collina, U. rosella).
Fig. 5: Correlation of indices for troglomorphic characters with the maximum age of inhabited cave formations (O. polyphemus, O. lorettae, O. gagnei, O. waikau, O. kalaupapae; S. pholetor (stonei), S. irvini (halos), S. baylissa; U. collina, U.
trends (troglomorphies) or regressive evolution. A ma- approach should aim at analyzing character evolution jor obstacle in this context is the poor comparability of in monophyletic groups where similar (morphological) characters across diferent taxonomic groups. A strict       pre-conditions or pre-adaptations for parallel evolution
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PATTERN AND PROCESS: EVOLUTION OF TROGLOMORPHy IN THE CAVE-PLANTHOPPERS OF AUSTRALIA AND HAwAI’I
may safely be assumed. while we are aware of these problems, we nevertheless found it useful to employ a quanti-fcation of troglomorphic characters in order to achieve at least a preliminary idea of the possible correlation be-tween troglomorphy and lineage age. we computed two ‘troglomorphy indices’ for all Hawaiian and Australian taxa from which data were available by using two charac-teristic troglomorphic characters in cavernicolous plan-thoppers: the reduction of eyes and the reduction of the tegmina. Eye reduction is apparently coupled with an ob-vious broadening of the vertex (see Fig. 2), so the index eye diameter: vertex width gives a clear statistical signal, ranging from 2-5 in epigean species with fully developed eyes to 0 in eyeless species. For the second index relative tegmina length we computed the absolute tegmina length: mesonotum width. Values ranges from 4.6-5.5 in epigean and from 1.1 to 5.3 in facultative and obligatory cavernicolous species.
Te conspicuous diferences between epigean and cave-dwelling species are clearly refected in both indices (Fig. 4). If the data are plotted against the maximum age
of the cave species, the a priori expectation is a clear negative correlation, at least for the Hawaiian taxa: the oldest cave lineages should exhibit the highest degree of troglo-morphy. In contrast, we surprisingly found a weak (not signifcant) positive trend (Fig. 5). Te same unexpected trends are seen in the Australian planthopper species.
Our results presented here, although preliminary, do not provide any evidence for cave-adaptation as a gradual orthogenetic process. Instead, we rather postu-late that founder efects indeed play an important role in the origin of cave species. A correlation of the observed trends with particular ecological parameters of the cave environment cannot be excluded based on our data, but clearly this hypothesis needs further testing, especially in respect to selection pressures exerted by the conditions in high stress environments (such as caves) (Hoch & How-arth 1989b, Howarth 1993). we can conclude with some certainty, however, that even in closely related species the degree of troglomorphy cannot be employed to infer the phylogenetic age of the cave lineages.
ACKNOwLEDGEMENTS
we thank Dr. Tomas von Rintelen, Museum für Naturkunde der Humboldt-Universität zu Berlin, for helpful discus-sions and many useful suggestions.
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