Strukturierungsmechanismen der Crematogaster-Macaranga Ameisen-Pflanzen Assoziationen : ein kombinierter ökologischer und phylogenetischer Ansatz

One of the most species-rich ant-plant mutualisms worldwide is the palaeotropical Crematogaster-Macaranga system. The pioneer-tree genus Macaranga (Euphorbiaceae) is mainly inhabited by at least nine specific species of Crematogaster (Myrmicinae), of which eight belong to the subgenus Decacrema, as well as several species of Camponotus (Formicinae). Ant species are not randomly distributed among the Macaranga host plants but distinct patterns of associations have been found (Fiala et al., 1999 and references cited therein). The specificity of the associations is maintained in spite of common sympatric distribution of several host-plant species. Associations are, however, usually not species-specific and especially the Decacrema ants, that are the focus of this study, usually colonize several host plant species each. In this study I used a combined approach of ecological data as well as phylogenetic data based on mitochondrial DNA sequences in order to elucidate the factors determining the patterns found in the associations and the evolution of this mutualistic system between the specific Decacrema ant partners and their Macaranga host plants. Life history traits of seven different morphospecies found on the most common Macaranga host plants were compared and colony development was followed from colony founding on saplings to adult trees. Temporal variability of the associations between Decacrema ants and their respective host plants was also examined. Associations between Crematogaster ants of the subgenus Decacrema and their Macaranga host plants were found to be stable over periods of time, long enough to enable reproduction of the ant colony and (in most cases) the host plants, too. Life-expectancy of the ant colony seems to be shorter than that of the host plant in general. All adult trees still provide nesting space as well as food for the ants. Colonies from different morphospecies differed in longevity, the onset of alate production, queen number and mode of colony founding. The examined Decacrema species could be placed into two groups according to their life-history traits as well as on morphological grounds: The decamera-group and the captiosa-group, each named after one species that could be synonymized with one morphospecies included in the group. Members of the captiosa-group have larger colonies, presumably with a longer life-span, and a later onset of reproduction compared to the decamera-group. Additionally, queens of the captiosa-group found colonies on saplings as well as in the crown region of bigger trees, whereas queens of the decamera-group found colonies on saplings and small treelets only. Queens belonging to the captiosa-group are brown with relatively large eyes (= 1/3 of the head length), whereas queens from the decamera-group are smaller in size, are dark brown to black in colour and have smaller eyes (< 1/3 of the head length). On some of the host plants examined in this study lifespan of the host plant and their specific ant partners seemed to be well matched whereas on others an ontogenetic succession of specific Decacrema partner ants was found, when host plants were abandoned due to the death of comparatively short-lived ant colonies, usually from species belonging to the decamera-group. Ant-partners of saplings or young plants often differed from specific partner ants found on bigger trees. Only species belonging to the captiosa-group were found to re-colonize the crown region of adult trees, thus facilitating a change of ant species, when longlived host plant species were colonized by relatively short-lived species from the decamera-group first. When long -lived host plants were colonized by long-lived species from the captiosa-group associations were stabler: I did not find any temporal variation in ant-inhabitants then. Life-span of the ant colony as well colony founding behaviour of the different partner ant species therefore play an important role for these ontogenetic changes and the specificity of the associations over time. For the host plant the ontogenetic changes have a strong impact as uninhabited host plants that are not patrolled by workers of specific ant partners suffer higher herbivore damage. Uninhabited host plants may also be colonized by unspecific arboreal ants that only make use of the nesting space and/ or food offered by the plant but do not confer protection against herbivores. Stable associations with a specific ant partner are therefore most beneficial for the host plants. Usually ant colonies are monogynous, but changes in the colony structure were found locally in two Decacrema species. I found colonies that turned secondarily polygynous, possibly after the death of the original founding queen. Secondary polygyny therefore can prolong the life-span of the antcolony on its host plant, leading to a parallel life-history and stable association as it was the case in Macaranga bancana-Crematogaster captiosa. However, in the other association (Macaranga hypoleuca-Crematogaster cf. decamera) life-expectancy of the ant-colony is still much shorter than that of its host plant species, leading to a change in the specific ant partner at a later stage. Pleometrotic foundress associations that directly led to polygynous colonies in one species were also found locally, a phenomenon hardly ever reported from ants in general. Foundress associations were found to be more successful in establishing colonies than single queens. I found indications that this change in colony founding behaviour might be due to interspecific competition for the same host plant species with another Decacrema species specific to Macaranga. For the phylogenetic analysis partial mitochondrial cytochrome oxidase I and II were sequenced and Neighbor-Joining, Maximum Parsimony, Maximum Likelihood as well as Bayesian analyses were performed. The four different analyses yielded phenetic as well as phylogenetic trees that all had a similar topology. Ants of the subgenus Decacrema formed a monophyletic clade, indicating a single colonization event at the beginning of the Macaranga-Decacrema symbiotic system. In the phylogenetic analysis the decamera-group as well as the captiosa-group were confirmed and clearly separated from each other. However, two species that would have been placed into the decamera-group, due to morphological as well as life-history traits, formed a third separate clade within the Decacrema. These two species (msp. 7- group) as well as the decamera-group came out as the basal groups in the phylogenetic analysis. Thus, life -history traits of these two groups (relatively small colonies, early onset of alate production, colony founding in ground region only) would be the ancestral state for Macarangaassociated ants of the subgenus Decacrema. Changes in colony structure, like secondary polygyny, were found in the captiosa- as well as the decamera-group and are therefore independent of the affiliation within the phylogeny. I did not find evidence for strict cocladogenesis between the subgenus Decacrema and their Macaranga host-plants, although ecological interactions between the two partner groups are close and associations can be rather specific. The phylogenies presented here, along with the known association patterns indicate that host-shifting of the ants is common in some of the species, opening the possibility of sympatric speciation as a result of increased host usage. Additionally, the considerable geographic substructuring found in the phylogenetic trees suggests that allopatric speciation has played a major role in diversification of the Decacrema ants.Das paläotropische Crematogaster-Macaranga System bildet eines der weltweit artenreichsten mutualistischen Ameisen-Pflanzen Interaktionssysteme. Es gibt knapp dreißig myrmekophytischen Arten innerhalb der Pionierpflanzengattung Macaranga (Euphorbiaceae), welche hauptsächlich von mindestens neun, für diese Pflanzen spezifischen, Ameisen der Gattung Crematogaster (Myrmicinae) besiedelt werden. Die Taxonomie der Ameisen ist jedoch nur teilweise geklärt, weshalb ein Großteil der Arten bisher nur in Morphospezies untergliedert wurde. Acht dieser neun Morphospezies gehören der Untergattung Decacrema an. Zusätzlich sind einige Macaranga-Arten mit Ameisen der Gattung Camponotus (Formicinae) assoziiert, die ebenfalls spezifisch für diese Wirtspflanzengattung sind. Die Ameisenarten zeigen keine stochastische Verteilung bei der Besiedlung der Macaranga-Wirtspflanzen. Stattdessen finden sich distinkte Verteilungsmuster und spezifische Assoziationen zwischen bestimmten Ameisen- und Pflanzenarten (Fiala et al., 1999 und die darin zitierte Literatur). Diese Verteilungsmuster der Ameisen auf ihren Wirtspflanzen bleiben trotz des häufig sympatrischen Vorkommens mehrerer Macaranga-Arten bestehen. Die Assoziationen sind jedoch größtenteils nicht im engeren Sinn artspezifisch, daß eine Ameisenart ausschließlich eine einzige Wirtspflanzenart besiedelt, bzw. jede Pflanzenart ausschließlich mit einer einzigen Ameisenart assoziiert ist. Gerade die Arten der Untergattung Decacrema, mit denen sich diese Studie hauptsächlich beschäftigt, besiedeln meist mehrere Wirtspflanzenarten. Bisher ist sehr wenig darüber bekannt, welche Faktoren für die - vor allem auf Keimlingen und Jungpflanzen nachgewiesenen -Besiedlungsmuster verantwortlich sind und ob und wie lange diese auf größeren Wirtspflanzen erhalten bleiben. In der vorliegenden Arbeit habe ich deshalb die Strukturierungsmechanismen der Crematogaster-Macaranga Assoziationen auf zwei verschiedenen Ebenen untersucht: sowohl die Stabilität und Aufrechterhaltung der rezenten Beziehungsmuster als auch mögliche historische Gründe für deren Entstehen. Hierfür habe ich zunächst vergleichend die Dynamik dieser Assoziationen von Jungpflanzen bis hin zu adulten Bäumen untersucht, als auch Aspekte der Evolution des Systems. Für die Untersuchung der Evolution dieses Systems wurde eine Verwandschaftsanalyse der Ameisen der Untergattung Decacrema auf molekularer Ebene mit mitochondrialer DNA von Cytochrom-Oxidase I und II durchgeführt. Um mögliche historische Faktoren aufzudecken, die zu diesen Besiedlungsmustern geführt haben könnten, wurden die Ergebnisse der dieser Verwandtschaftsanalyse dann mit dem vorhandenen Stammbaum der Wirtspflanzenarten verglichen und speziell vor dem Hintergrund der ökologischen Daten diskutiert. Zudem wurden life-history traits (Charakteristika der Lebensgeschichte) von sieben verschiedenen Morphospezies, die auf den häufigsten Macaranga-Arten vorkommen vergleichend untersucht, und deren Kolonieentwicklung von der Koloniegründung auf Keimlingen bis hin zu adulten Bäumen erfasst. Assoziationen zwischen Ameisen der Untergattung Decacrema und ihren Macaranga-Wirtspflanzen waren zumindest so lange stabil, daß sowohl die Ameisen als auch in den meisten Fällen die Wirtspflanzen ihre reproduktive Phase erreichen konnten. Generell scheint die Lebensdauer der Ameisenkolonien kürzer zu sein, als die ihrer Wirtspflanzen. Alle Wirtspflanzen behielten ihre Attraktivität für Ameisen bei, indem sie auch im Adultstadium Nahrung und Nistraum zur Verfügung stellten. Kolonien der sieben untersuchten Morphospezies unterschieden sich in Lebensdauer, Beginn der Reproduktionsphase, Gynie und der Art der Koloniegründung. Die untersuchten Decacrema-Arten können mit Hilfe der Unterschiede in ihren life-history traits, als auch nach ihrer Morphologie in zwei Gruppen unterteilt werden: die decamera-Gruppe und die captiosa- Gruppe, jeweils benannt nach einer beschriebenen Art innerhalb dieser Gruppe, die mit einer in vorherigen Arbeiten verwendeten Morphospezies synonymisiert werden konnte. Arten innerhalb der captiosa-Gruppe haben, im Vergleich zu Arten der decamera-Gruppe, größere Kolonien, wahrscheinlich eine längere Lebensdauer und erreichen ihre reproduktive Phase später. Königinnen der captiosa-Gruppe können sowohl in Keimlingen als auch in der Kronenregion größerer Bäume Kolonien gründen, wohingegen gründende Königinnen der decamera-Gruppe ausschließlich bodennah auf Keimlingen und kleineren Bäumen gefunden wurden. Zudem unterscheiden sich die Königinnen von Arten der beiden Gruppen in ihrer Morphologie: Königinnen der captiosa-Gruppe sind bräunlich gefärbt und haben relativ große Komplexaugen (= 1/3 der Kopflänge), während Königinnen der decamera-Gruppe relativ klein und dunkelbraun bis schwarz gefärbt sind. Zudem haben sie relativ kleinere Komplexaugen (< 1/3 der Kopflänge). Bei einigen der untersuchten Assoziationen scheinen die Lebensdauer der Wirtspflanzen und ihrer spezifischen Ameisenkolonien gut aufeinander abgestimmt zu sein, wohingegen bei anderen eine ontogenetische Sukzession von verschiedenen spezifischen Decacrema-Arten gefunden wurde. Dies war vor allem dann der Fall, wenn Wirtspflanzen zunächst von Arten der decamera-Gruppe besiedelt waren, deren relativ kurzlebige Kolonien früh abstarben. Da nur Königinnen von Arten der captiosa- Gruppe auch in der Kronenregion größerer Bäume Kolonien gründen können, kommt es in diesen Fällen zu einem Wechsel in der Ameisenbesiedlung. Die Assoziationen zwischen langlebigen Wirtspflanzenarten und den langlebigeren Kolonien von Arten der captiosa-Gruppe waren stabiler. Bei diesen Assoziationen wurde kein Wechsel der spezifischen Ameisenart über die Zeit festgestellt. Die Lebensdauer der Ameisenkolonien sowie das Gründungsverhalten von Königinnen spielen deshalb eine wichtige Rolle bei der ontogenetischen Sukzession der Ameisenbesiedler auf den Wirtspflanzen. Die ontogenetischen Wechsel in der Besiedlung sind für die Wirtspflanzen von großer Bedeutung, denn in Zeiten, in denen sie nicht von einer intakten und der Größe der Pflanze entsprechenden Kolonie bewohnt sind, fehlt ihnen der Schutzeffekt der Ameisen gegen Herbivorie. Unbewohnte Bäume können sowohl von spezifischen Decacrema-Arten der captiosa-Gruppe wiederbesiedelt werden als auch von unspezifischen arborealen Ameisenarten, die zwar den angebotenen Nistraum und/ oder Nahrung nutzen, jedoch keinen Herbivorieschutz gewähren. Aus diesem Grund sind möglichst stabile Assoziationen mit einer spezifischen Decacrema-Art für die Wirtspflanze von Vorteil. Kolonien der Decacrema-Arten sind normalerweise monogyn. Lokal wurden jedoch Kolonien von zwei Arten entdeckt, di

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