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.

Lack of interruption of the gene network underlying wing polyphenism in an early-branching ant genus.

Ants evolved about 140 million years ago and have diversified into more than 15,000 species with tremendous ecological and morphological diversity, yet evolution of the gene regulatory networks (GRNs) underlying this diversification remains poorly understood. Wing polyphenism, the ability of a single genome to produce either winged or wingless castes during development in response to environmental cues, is a nearly universal feature of ants. The underlying wing GRN is evolutionarily labile in worker castes of phylogenetically derived species: it is conserved in winged castes but interrupted at different points in wingless castes of different species. However, it remains unknown whether the wing GRN is interrupted in wingless castes of species from early branching lineages, and if so, whether it is interrupted at similar locations in worker castes of derived species. We therefore used in situ hybridization to assay the expression of nine genes in the wing GRN in wing imaginal discs of larvae from two species from the early branching ('basal') genus Mystrium. These species possess two castes each: Mystrium rogeri has winged queens and wingless workers, and M. oberthueri has wingless queens and wingless workers. In contrast to derived species, we found no evidence of interruption points in the wing GRN kernel of wingless castes. Our finding supports: (1) a "phylogenetic ladder model" of wing GRN evolution, where interruption points move further upstream in the wing GRN as ant lineages become more derived; and (2) that evolutionary lability of the GRN underlying wing polyphenism originated later during ant evolution.

Effect of temperature and social environment on worker size in the ant Temnothorax nylanderi.

Warm temperatures decrease insect developmental time and body size. Social life could buffer external environmental variations, especially in large social groups, either through behavioral regulation and compensation or through specific nest architecture. Mean worker size and distribution of worker sizes within colonies are important parameters affecting colony productivity as worker size is linked to division of labor in insect societies. In this paper, we investigate the effect of stressful warm temperatures and the role of social environment (colony size and size of nestmate workers) on the mean size and size variation of laboratory-born workers in the small European ant Temnothorax nylanderi. To do so, we reared field-collected colonies under medium or warm temperature treatments after having marked the field-born workers and removed the brood except for 30 first instar larvae. Warm temperature resulted in the production of fewer workers and a higher adult mortality, confirming that this regime was stressful for the ants. T. nylanderi ants followed the temperature size rule observed in insects, with a decreased developmental time and mean size under warm condition. Social environment appeared to play an important role as we observed that (i) larger colonies buffered the effect of temperature better than smaller ones (ii) colonies with larger workers produced larger workers whatever the rearing temperature and (iii) the coefficient of variation of worker size was similar in the field and under medium laboratory temperature. This suggests that worker size variation is not primarily due to seasonal environmental fluctuations in the field. Finally, we observed a higher coefficient of variation of worker size under warm temperature. We propose that this results from a disruption of social regulation, i.e. the control of nestmate workers over developing larvae and adult worker size, under stressful conditions.