Primary cell cultures from the single-chromosome ant Myrmecia croslandi.

The number of chromosomes varies tremendously across species. It is not clear whether having more or fewer chromosomes could be advantageous. The probability of non-disjunction should theoretically decrease with smaller karyotypes, but too long chromosomes should enforce spatial constraint for their segregation during the mitotic anaphase. Here, we propose a new experimental cell system to acquire novel insights into the mechanisms underlying chromosome segregation. We collected the endemic Australian ant Myrmecia croslandi, the only known species with the simplest possible karyotype of a single chromosome in the haploid males (and one pair of chromosomes in the diploid females), since males are typically haploid in hymenopteran insects. Five colonies, each with a queen and a few hundreds of workers, were collected in the Canberra district (Australia), underwent karyotype analysis to confirm the presence of a single pair of chromosomes in worker pupae, and were subsequently maintained in the laboratory in Paris (France). Starting from dissociated male embryos, we successfully conducted primary cell cultures comprised of single-chromosome cells. This could be developed into a unique model that will be of great interest for future genomic and cell biology studies related to mitosis.

The role of attractive and repellent scene memories in ant homing (Myrmecia croslandi).

Solitary foraging ants rely on vision when travelling along routes and when pinpointing their nest. We tethered foragers of Myrmecia croslandi on a trackball and recorded their intended movements when the trackball was located on their normal foraging corridor (on-route), above their nest and at a location several metres away where they have never been before (off-route). We found that at on- and off-route locations, most ants walk in the nest or foraging direction and continue to do so for tens of metres in a straight line. In contrast, above the nest, ants walk in random directions and change walking direction frequently. In addition, the walking direction of ants above the nest oscillates on a fine scale, reflecting search movements that are absent from the paths of ants at the other locations. An agent-based simulation shows that the behaviour of ants at all three locations can be explained by the integration of attractive and repellent views directed towards or away from the nest, respectively. Ants are likely to acquire such views via systematic scanning movements during their learning walks. The model predicts that ants placed in a completely unfamiliar environment should behave as if at the nest, which our subsequent experiments confirmed. We conclude first, that the ants' behaviour at release sites is exclusively driven by what they currently see and not by information on expected outcomes of their behaviour; and second, that navigating ants might continuously integrate attractive and repellent visual memories. We discuss the benefits of such a procedure.

Mapping the navigational knowledge of individually foraging ants, Myrmecia croslandi.

Ants are efficient navigators, guided by path integration and visual landmarks. Path integration is the primary strategy in landmark-poor habitats, but landmarks are readily used when available. The landmark panorama provides reliable information about heading direction, routes and specific location. Visual memories for guidance are often acquired along routes or near to significant places. Over what area can such locally acquired memories provide information for reaching a place? This question is unusually approachable in the solitary foraging Australian jack jumper ant, since individual foragers typically travel to one or two nest-specific foraging trees. We find that within 10 m from the nest, ants both with and without home vector information available from path integration return directly to the nest from all compass directions, after briefly scanning the panorama. By reconstructing panoramic views within the successful homing range, we show that in the open woodland habitat of these ants, snapshot memories acquired close to the nest provide sufficient navigational information to determine nest-directed heading direction over a surprisingly large area, including areas that animals may have not visited previously.

Anesthesia disrupts distance, but not direction, of path integration memory.

Solitary foraging insects, such as ants, maintain an estimate of the direction and distance to their starting location as they move away from it, in a process known as path integration. This estimate, commonly known as the "home vector," is updated continuously as the ant moves1-4 and is reset as soon as it enters its nest,5 yet ants prevented from returning to their nest can still use their home vector when released several hours later.6,7 This conjunction of fast update and long persistence of the home vector memory does not directly map to existing accounts of short-, mid-, and long-term memory;2,8-12 hence, the substrate of this memory remains unknown. Chill-coma anesthesia13-15 has previously been shown to affect associative memory retention in fruit flies14,16 and honeybees.9,17,18 We investigate the nature of path integration memory by anesthetizing ants after they have accumulated home vector information and testing if the memory persists on recovery. We show that after anesthesia the memory of the distance ants have traveled is degraded, but the memory of the direction is retained. We also show that this is consistent with models of path integration that maintain the memory in a redundant Cartesian coordinate system and with the hypothesis that chill-coma produces a proportional reduction of the memory, rather than a subtractive reduction or increase of noise. The observed effect is not compatible with a memory based on recurrent circuit activity and points toward an activity-dependent molecular process as the basis of path integration memory.

Looking and homing: how displaced ants decide where to go.

We caught solitary foragers of the Australian Jack Jumper ant, Myrmecia croslandi, and released them in three compass directions at distances of 10 and 15 m from the nest at locations they have never been before. We recorded the head orientation and the movements of ants within a radius of 20 cm from the release point and, in some cases, tracked their subsequent paths with a differential GPS. We find that upon surfacing from their transport vials onto a release platform, most ants move into the home direction after looking around briefly. The ants use a systematic scanning procedure, consisting of saccadic head and body rotations that sweep gaze across the scene with an average angular velocity of 90° s(-1) and intermittent changes in turning direction. By mapping the ants' gaze directions onto the local panorama, we find that neither the ants' gaze nor their decisions to change turning direction are clearly associated with salient or significant features in the scene. Instead, the ants look most frequently in the home direction and start walking fast when doing so. Displaced ants can thus identify home direction with little translation, but exclusively through rotational scanning. We discuss the navigational information content of the ants' habitat and how the insects' behaviour informs us about how they may acquire and retrieve that information.