Chemistry of the postpharyngeal gland secretion and its implication for the phylogeny of IberianCataglyphis species (Hymenoptera: Formicidae)

Intersexual and body size-related variation in chemical constituents from feces and cloacal products involved in intraspecific communication of a fossorial amphisbaenian

Background

Many animals rely on chemical cues for intraspecific communication. This is especially important in fossorial animals because visual restrictions of the underground environment limit the opportunities for visual communication. Previous experiments showed the ability of the amphisbaenian Trogonophis wiegmanni to discriminate between several categories of conspecifics based on chemical cues alone. However, in contrast with many other reptile species, T. wiegmanni does not have external secretory glands, but uses uncharacterized secretions from the cloaca in intraspecific chemosensory communication.

Methods

Using gas chromatography-mass spectrometry (GC-MS), we analyzed the lipophilic compounds from feces and cloacal products freshly extracted from the cloaca of male and female T. wiegmanni. We identified and estimated relative proportions of the compounds found, and tested for intersexual and body-size related differences.

Results

We found a total of 103 compounds, being some steroids (mainly cholesterol and cholestanol), some alkanes and squalene the most abundant and frequent. Further, we found intersexual differences, with males, especially larger ones, having higher proportions of several alkanes between C₁₃ and C₂₄ and of squalene than females, which had higher proportions of several steroids and also of nonacosane and methylnonacosane than males. We compared these findings with secretions of other animals and discuss the potential role of these compounds and their variations in intraspecific communication of amphisbaenians.

Cytonuclear incongruences hamper species delimitation in the socially polymorphic desert ants of the Cataglyphis albicans group in Israel

Assessing whether behavioural, ecological or geographical factors trigger population divergence provides key insights into the biological processes driving speciation. Recent speciation in restricted geographic area without obvious ecological barriers prompts the question of the behavioural mechanisms underlying species divergence. In this context, we investigated phylogenetic relationships in the Cataglyphis albicans desert ant complex in Israel. We first determined accurate species delimitation using two mitochondrial and six nuclear genes, as well as 11 microsatellite markers to investigate cryptic species in this group, assessing reduction in gene flow between populations. We then investigated whether different species in this group exhibit distinct reproductive strategies, inferring social structure and queen-mating frequency in each species uncovered. Our findings highlight the presence of at least six distinct Cataglyphis albicans species in the restricted range of Israel; four of them co-occur in a 50 × 50 km area in North Negev, while two are endemic from there. However, our results reveal incongruences between nuclear and mitochondrial clustering, which complicate species identification and preclude the exclusive use of mtDNA to confidently delimit species in this group. Finally, we show that the different species of the C. albicans group in Israel exhibit quite similar reproductive strategies with most of them having colonies headed by a single queen mated with several males; colonies of one species were, however, headed by several queens. Overall, this weak variation across species thereby unlikely represents the main evolutionary force behind speciation of these sympatric species. We then discuss the potential evolutionary processes that underlie speciation in this group in the absence of clear geographical or ecological barriers.

How do cuticular hydrocarbons evolve? Physiological constraints and climatic and biotic selection pressures act on a complex functional trait

Cuticular hydrocarbons (CHCs) cover the cuticles of virtually all insects, serving as a waterproofing agent and as a communication signal. The causes for the high CHC variation between species, and the factors influencing CHC profiles, are scarcely understood. Here, we compare CHC profiles of ant species from seven biogeographic regions, searching for physiological constraints and for climatic and biotic selection pressures. Molecule length constrained CHC composition: long-chain profiles contained fewer linear alkanes, but more hydrocarbons with disruptive features in the molecule. This is probably owing to selection on the physiology to build a semi-fluid cuticular layer, which is necessary for waterproofing and communication. CHC composition also depended on the precipitation in the ants' habitats. Species from wet climates had more alkenes and fewer dimethyl alkanes than those from drier habitats, which can be explained by different waterproofing capacities of these compounds. By contrast, temperature did not affect CHC composition. Mutualistically associated (parabiotic) species possessed profiles highly distinct from non-associated species. Our study is, to our knowledge, the first to show systematic impacts of physiological, climatic and biotic factors on quantitative CHC composition across a global, multi-species dataset. We demonstrate how they jointly shape CHC profiles, and advance our understanding of the evolution of this complex functional trait in insects.

An integrative approach to untangling species delimitation in the Cataglyphis bicolor desert ant complex in Israel

Although extensive research has been carried out on the desert ants in the genus Cataglyphis in recent years, some of the specific intra- and interspecific relationships remain elusive. The present study disentangles the phylogenetic relationships among the C. bicolor complex in Israel using an integrative approach based on genetic markers, morphometric measurements, and chemical analyses (cuticular hydrocarbons). Several species delimitation approaches based on four nuclear, two mitochondrial, and eleven microsatellite markers, as well as 16 body measurements and 56 chemical variables, were employed to deciphering the occurrence of cryptic species in our data set. Our findings support the occurrence of at least four distinct species in the C. bicolor group in Israel, one of which may be a complex of three more recent species. The findings confirm the distinctiveness of C. isis and C. holgerseni. They attest the presence of a recently discovered species, C. israelensis, in the central mountain ridge and the occurrence of another clade distributed from the Negev to the Mediterranean coast, comprising the species C. niger, C. savignyi, and C. drusus. Although these three species are separated on the basis of mtDNA, this subgrouping was not supported by any of the nuclear sequence markers nor by the microsatellite analysis. This genetic structure may thus either reflect a possible recent speciation, or a geographical structuring of a single species. Overall, using these different sources of evidence we locate our samples within a global phylogeny of the bicolor group and discuss the processes that underlie speciation in this group.

Combined hybridization and mitochondrial capture shape complex phylogeographic patterns in hybridogenetic Cataglyphis desert ants

Some species of Cataglyphis desert ants have evolved a hybridogenetic mode of reproduction at the social scale. In hybridogenetic populations, two distinct genetic lineages coexist. Non-reproductive offspring (workers) are hybrids of the two lineages, whereas sexual offspring (males and new queens) are produced by parthenogenesis and belong to the mother queen lineage. How this unusual reproductive system affects phylogeographic patterns and speciation processes remains completely unknown to date. Using one mitochondrial and four nuclear genes, we examined the phylogenetic relationships between three species of Cataglyphis (C. hispanica, C. humeya and C. velox) where complex DNA inheritance through social hybridogenesis may challenge phylogenetic inference. Our results bring two important insights. First, our data confirm a hybridogenetic mode of reproduction across the whole distribution range of the species C. hispanica. In contrast, they do not provide support for hybridogenesis in the populations sampled of C. humeya and C. velox. This suggests that these populations are not hybridogenetic, or that hybridogenesis is too recent to result in reciprocally monophyletic lineages on nuclear genes. Second, due to mitochondrial introgression between lineages (Darras and Aron, 2015), the faster-evolving COI marker is not lineage specific, hence, unsuitable to further investigate the segregation of lineages in the species studied. Different mitochondrial haplotypes occur in each locality sampled, resulting in strongly structured populations. This micro-allopatric structure leads to over-splitting species delimitation on mitochondrial gene, as every locality could potentially be considered a putative species; haploweb analyses of nuclear markers, however, yield species delimitations that are consistent with morphology. Overall, this study highlights how social hybridogenesis varies across species and shapes complex phylogeographic patterns.

Recent speciation and secondary contact in endemic ants

Gene flow is the main force opposing divergent selection, and its effects are greater in populations in close proximity. Thus, complete reproductive isolation between parapatric populations is not expected, particularly in the absence of ecological adaptation and sharp environmental differences. Here, we explore the biogeographical patterns of an endemic ant species, Cataglyphis floricola, for which two colour morphs (black and bicolour) coexist in parapatry throughout continuous sandy habitat in southern Spain. Discriminant analyses of six biometric measurements of male genitalia and 27 cuticular hydrocarbons reveal high differentiation between morphs. Furthermore, the low number of shared alleles derived from nuclear markers (microsatellites) between the morphs at their contact zone suggests the absence of recent gene flow. Mitochondrial DNA (COI) phylogenetic analysis and median-joining networks show that the black morph is basal to the bicolour morph, with unique haplotypes recovered for each morph. Mismatch distribution analysis and Bayesian skyline plots suggest that they are undergoing different demographic changes, with the bicolour and black morphs at demographic equilibrium and expansion, respectively. Thus, our results show complete reproductive isolation between the two colour morphs as evidenced from genetic, chemical and morphological data. We suggest that these divergence events could be explained by historical vicariance during the Pleistocene, in which reproductive traits experienced strong divergent selection between the morphs initiating or culminating speciation.

Evolution of cuticular hydrocarbon diversity in ants

The cuticular hydrocarbons (CHCs) of ants provide important cues for nest-mate and caste recognition. There is enormous diversity in the composition of these CHCs, but the manner in which this diversity has evolved is poorly understood. We gathered data on CHC profiles for 56 ant species, relating this information to their phylogeny. We deduced the mode of evolution of CHC profiles by reconstructing character evolution and then relating the number of changes in CHC components along each branch of the phylogeny to the length of the branch. There was a strong correlation between branch length and number of component changes, with fewer changes occurring on short branches. Our analysis thereby indicated a gradual mode of evolution. Different ant species tend to use specific CHC structural types that are exclusive of other structural types, indicating that species differences may be generated in part by switching particular biosynthetic pathways on or off in different lineages. We found limited, and contradictory, evidence for abiotic factors (temperature and rainfall) driving change in CHC profiles.

A review of ant cuticular hydrocarbons

We compared the published cuticular hydrocarbon (CHC) profiles of 78 ant species across 5 subfamilies. Almost 1,000 CHCs have been described for these species, composing 187 distinct homologous series and ten hydrocarbon groups. In descending order of occurrence were: n-alkanes > monomethylalkanes > dimethylalkanes > alkenes > dienes>> trimethylalkanes>> methylalkenes > methylalkadienes > trienes > tetramethylalkanes. Odd chain lengths and positions of methyl or double bonds at odd carbon numbers were far more numerous than even chain-length compounds or bond positions. Although each species possess its own unique pattern of CHCs, we found no association between CHC profile and phylogeny. The production of the biosynthetically complex compounds (e.g., methyl branched dienes) by the most primitive living ant suggests that the basic genetic architecture required to produce the rich diversity of CHCs was already present prior to their adaptive radiation. Unlike the ubiquitous n-alkanes and monomethylalkanes, there is a huge diversity of species-specific dimethylalkanes that makes them likely candidates for species and nest-mate discrimination signals.

Colony-specific hydrocarbons identify nest mates in two species of Formica ant

The possession of a colony identity is a fundamental property that underlies much animal behavior. In insect societies, it is widely accepted that nest-mate recognition cues are encoded within the cuticular hydrocarbons. Despite numerous studies over the past 30years, the identification of these nest-mate specific signatures is only just starting to occur. In this paper, we show two different methods by which nest-mate-specific signatures can be encoded within the hydrocarbon profile of two species of Formica ants. In F. exsecta, nest-mate-specific signatures rely on the distribution of chain lengths of a single type of hydrocarbon, various (Z)-9-alkenes, which are present in colony-specific proportions. In F. fusca, variation in nine different positional isomers of C(25)-dimethylalkanes is sufficient to produce unique colony profiles. By using this information alone, we correctly assigned 97 F. exsecta workers into their respective 20 colonies and 111 F. fusca workers into their respective 30 colonies. These two systems or variations of them may be expected to occur in many insect societies that have a strong colony identity.

Chemical basis of nest-mate discrimination in the ant Formica exsecta

Distinguishing nest-mates from non-nest-mates underlies key animal behaviours, such as territoriality, altruism and the evolution of sociality. Despite its importance, there is very little empirical support for such a mechanism in nature. Here we provide data that the nest-mate recognition mechanism in an ant is based on a colony-specific Z9-alkene signature, proving that surface chemicals are indeed used in ant nest-mate recognition as was suggested 100 years ago. We investigated the cuticular hydrocarbon profiles of 10 Formica exsecta colonies that are composed almost entirely of a Z9-alkene and alkane component. Then we showed that worker aggression is only elicited by the Z9-alkene part. This was confirmed using synthetic Z9-alkene and alkane blends matched to the individual colony profiles of the two most different chemical colonies. In both colonies, only glass beads with 'nest-mate' alkene profiles received reduced aggression. Finally, changing the abundance of a single Z9-alkene on live ants was shown to significantly increase the aggression they received from nest-mates in all five colonies tested. Our data suggest that nest-mate discrimination in the social insects has evolved to rely upon highly sensitive responses to relatively few compounds.

Novel wax esters and hydrocarbons in the cuticular surface lipids of the red harvester ant, Pogonomyrmex barbatus

The cuticular surface lipids of the red harvester ant, Pogonomyrmex barbatus, were found to contain minor amounts of novel wax esters, in addition to the major components, hydrocarbons. The wax esters ranged in carbon number from C19 to C31 and consisted of esters of both odd- and even-numbered alcohols and acids. Each wax ester with a given carbon number eluted at several different retention times indicating possible methyl branching in either the fatty acid or alcohol moiety, or in both moieties. Each eluting peak of wax esters consisted of a mixture of wax esters of the same carbon number in which the fatty acid moiety ranged from C8 to C18, and the alcohol moiety ranged from C8 to C17. Some wax esters were largely found on the head indicating they may be of a glandular origin. The hydrocarbons consisted of: n-alkanes, C23 to C33; odd-numbered n-alkenes, C27 to C35; and the major components, methyl-branched alkanes, C26 to over C49. Notable components of the methyl-branched alkanes were 2-methyltriacontane, and the novel trimethylalkanes with a single methylene between the first and second branch points, 13,15,19-trimethylhentriacontane and 13,15,21-trimethyltritriacontane.