Cardiocondyla: Heart Node Ants
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Cardiocondyla is an Old World genus of myrmicine ants with a broad distribution. At present 48 species are described, with an estimated total of more than 100 species around the world . The common name comes from the heart-shaped post-petiole in most species, which is much broader than the petiole. Cardiocondyla are small ants (1.5–3 mm worker body length) and have a three-segmented antennal club, propodeal spines, and a slender, elegant body shape. Cardiocondyla is believed to have originated somewhere in the Indomalayan or the Australasian region . Dating its origin is problematic because of its uncertain phylogenetic position within the myrmicine clade Crematogastrini but is estimated to be around 50mya .
Cardiocondyla typically have small colonies of a few dozen to a few hundred workers. The large majority of species nests in the soil, while several species of the C. argentea clade (provisional name, currently under investigation by B. Seifert) nest under rocks or in rock crevices, and only species of the C. wroughtonii clade have adopted an arboreal lifestyle, inhabiting ephemeral small nest sites under bark and in hollow twigs, etc. The workers usually forage singly for small food items and fluids. All ground-nesting species studied so far are characterized by a peculiar disorienting homing behavior of single foragers, which has tested the patience of many collectors after baiting the ants with small cookie crumbs. Due to this kind of behavior, several authors have referred to Cardiocondyla as “sneaking ants.” These characteristic deceptive return trips likely evolved to hide the inconspicuous nest entrance. Neither pheromone-enhanced trails nor cooperative food retrieval has been documented. However, workers of species from all major lineages within Cardiocondyla (i.e., C. obscurior, C. thoracica, C. venustula, C. emery, C. minutior, C. kagutsuchi, and C. elegans) have been observed recruiting nestmates by tandem running, using poison gland secretions, similar to Leptothorax species. Due to their small size, inconspicuous nature and potent venom, they are ignored by larger ant species such as Pheidole and Solenopsis. Indeed, many students have witnessed painful but short-lasting stings from several Cardiocondyla species despite their minute size.
Several Cardiocondyla species (C. obscurior, C. wroughtonii, C. emery, C. minutior, C. kagutsuchi, C. venustula) are found in warm climates around the globe and have become what one could call “cryptotramps,” i.e., introduced species that thrive in disturbed habitat but are mostly invisible to our eyes. Cardiocondyla is well preadapted to a tramp lifestyle because of intranidal mating of wingless males with closely related females, thus forming new populations from tiny propagules. This has been called “perpetual consanguineous reproduction”  – what an elegant expression – and evolved along with the nearly universal wing polyphenism in females and a novel developmental switch that gave rise to both winged and wingless males . Wingless males have been shown to play a prominent role in all aspects of the colony cycle. Not only are they at the core of competition over sex allocation under local mate competition  or reproductive dominance , but they also have direct effects on queen fitness . The phylogeny of Cardiocondyla suggests that wingless males evolved once at the base of the genus  and that the winged morph was lost several times independently. This is consistent with the highly variable nature of male phenotypic plasticity within the genus, such that several species produce wingless males exclusively, while others produce both morphs, and some species even consistently produce intermorphs between winged and wingless males. In all cases, these males facilitate mating between close kin and have greatly accelerated the evolution of the genus.
At the developmental and genetic levels, it has been suggested that polyphenism in males and females in C. obscurior evolved through co-option of ancient genetic sex differentiation pathways . These include the terminal sex differentiation transcription factor, doublesex, which is known to control a large suite of developmental toolkit genes. Furthermore, early third instar larvae of C. obscurior show a large morph-specific gene expression bias. In particular, wingless males at this stage are characterized by an enriched expression of genes associated with larval and imaginal disc development . In addition, expression plasticity is positively correlated with evolutionary rate . Ants of the genus Cardiocondyla are exceptional in their extent of realized phenotypic plasticity and thus make a promising model for dissecting molecular and genomic differences underlying the evolution of novel developmental switches through genetic accommodation.
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