×The structural and functional features that animals develop in order to survive and thrive in their environment have long drawn the attention of the scientific community. Regarding insular ecosystems, the unique physical-natural conditions prevailing on them, the strictly defined by the seas boundaries and the more simplified systems induce the diversification and evolution of island biotas with a number of ways.
The main aim of the present research was first, to investigate the effect of natural selection on the ecology, physiology and general biology of ectothermic animals, such as lizards and second, to examine how natural selection can shape morphological, functional and behavioral traits. To this aim, we studied insular and mainland populations of the lizard species Lacerta trilineata (Sauria: Lacertidae). The green lizards of the genus Lacerta comprises nine allopatric species, four of which occur in Greece, while only L. trilineata is distributed in both continental and insular Greek region. The latter feature makes L. trilineata an ideal organism to study the phenotypic plasticity of physiological and morphological traits. How and with what adaptations was its current distribution formed? How did insular and mainland populations have diversified to the different environmental conditions? Are there historical or ecological causes that led to this diversification? These are some of the subjects that this PhD Thesis was dealt with. Through a comparative framework, that contained several L. trilineata populations we tried to address these issues.
The resolved phylogeny revealed that the first branch-off of the Greek green lizards occurred during the middle Miocene (13.91 Mya) and led to the separation of the L. viridis/bilineata group from the species L. agilis and L. trilineata. The next major cladogenetic event took place at late Miocene (11.65 Mya) and led to the separation of the latter two species. Regarding the clade of L. trilineata our results support the existence of six distinct lineages. The high intraspecific genetic divergences between these lineages, which are comparable to that recorded among lacertid species, raises questions regarding its taxonomic status and whether or not it is a single species. L. trilineata seems to originate and diverge in western Anatolia. Given the phylogeny of L. trilineata in the Aegean and the geological history of the Greek area, we assume that the ancestral form of this species invaded the Greek area from Anatolia before the complete formation of the MAT (~9 Mya). The forming of this barrier gave the two groups of L. trilineata lineages (west and east of the trench). Within each group, some other vicariant and dispersal events as well as the raise of the Mediterranean Sea at the end of the last glacial period during Holocene have resulted in the present distribution of the six evolutionary lineages of L. trilineata.
In order to approach the thermal biology of this species, we studied two mainland and two island populations and determined three different thermal values for each population: lizard`s temperature under normal conditions in the field (body temperature), the temperature that a lizard may achieve in a special designed thermal gradient in the lab (selective temperature) and the potential temperatures that an animal could reach in its biotope (operative temperatures). From the correlation of these three values we tried to describe the strategy of thermoregulation that each population has adopted. The comparison between mainland and island populations revealed that both populations have adapted to their thermal environment, supporting the `labile` view on the evolution of thermal physiology for this species. In particular, insular lizards thermoregulate less effectively because of the more benign, in terms of thermal quality, environment and have lower thermal preferenda. Mainland lizards on the other hand, showed higher effectiveness as a result of the more demanding environment. However, predation regime seems to restrict the ability of the latter to achieve body temperatures close to their set-point range.
To investigate the trophic ecology of the Balkan green lizard, we removed the digestive track of preserved specimens as well as collected the fecal material from live lizards, and we correlated the trophic data with morphological and functional traits. Our findings support that L. trilineata is a widely foraging predator, feeding on numerous invertebrate taxa, while Coleoptera and Orthoptera represent the predominant prey group. Ontogeny, sex, season and habitat affect the feeding ecology of this species. The seasonal variation in the diet of L. trilineata corroborate that the species is an opportunistic predator. However, juveniles followed a discrete feeding pattern, which indicates an ontogenetic shift. During their growth, lacertids undergo significant changes in the size, shape and performance of their feeding apparatus. A complementary underlying reason should be sought in the way adults exploit their environment and the areas that usually occupy. Finally, regarding the observed differences in diet composition between sexes as well as island and mainland populations, these probably reflect adaptations through sexual and natural selection. The more robust heads of male lizards allows them to develop a greater bite force and consequently to feed on harder prey items. In addition, the food scarcity prevailing in the Mediterranean islands offers little choice to insular lizards. Thus islanders have to take advantage of every available food resource in order to survive even plant material, which they achieved by increasing the head depth and width as well as bite force, compared to their mainland counterparts.
Differences on feeding ecology usually affect several aspects of the digestive system such as physiology and morphology. In the study of the digestive procedure we tried to obtain a qualitative approach of the digestion of the main organic compounds (lipids, sugars and proteins). Islanders deviated from their mainland peers increasing the digestive tract length and the frequency of cecal valves in the hindgut. However, an increase in the digestive tract length and the frequency of cecal valves might cause an increase of gut passage time and, thus, food retention times. In other words, enzymes could have more time to act, which maximize the rate of energy intake and the digestive efficiency. We believe that these changes on the digestive procedure could be interpreted as adaptations for the limited food availability prevailing on islands.
Finally, given that the ability of an animal to successfully colonize new habitats is mainly determined by its ability to cope with pathogens of the environment, we studied the immune response in association with the genetic diversity of the major histocompatibility complex (MHC) class I. For the study of cell-mediated response we used two methodological approaches: (a) the mixed lymphocyte reaction (MLR) and (b) the phytohemagglutinin-induced skin swelling test (PHA). Our finding revealed that the genetic boundaries of islands and the food scarcity prevailing on them caused decline in the genetic divergence at the MHC genes and in parasite tolerance. Nonetheless, the significant importance of balancing selection against genetic drift, in shaping MHC polymorphism, has lead to an increase on the nucleotide diversity of the remaining MHC alleles. As a result and besides the reduce parasite tolerance of islanders, the latter demonstrated high cell mediated response, which is comparable to their mainland peers.
To sum up, it appears that island populations of L. trilineata have adapted to the different biotic and abiotic environmental conditions of islands, which have lead to the development of unique morphological, physiological and behavioral traits.
