The invasive fire ant, Solenopsisinvicta, was first documented in Taiwan in 2003 and is currently found in large populations in greater Taipei, Taoyuan, Chiayi, Miaoli and Hsinchu counties with sporadic appearances elsewhere. These aggressive ants, which are named after their painful and burning stings induced by toxic venom, pose a serious threat to Taiwan ecology as well as to people, whose responses to being stung range from developing an annoying itch to death, if the victim has an allergic reaction. In Taiwan and elsewhere, efforts to contain or eradicate the fire ant have been hampered by the difficulty in finding all the colonies in a given location (for pesticide treatment) combined with the high reproductive rate of fire ant colonies, which sometimes reaches thousands of males or queens per colony.

　　The fire ant genome was sequenced in 2011 allowing insights into the types and numbers of genes it possesses. In this study, a closer look at the genes of the fire ant has revealed a ‘social chromosome’ that seems to offer an explanation as to why some fire ant colonies shelter only one single queen while others shelter many. Such a discovery could prove to be helpful in finding genetic solutions for controlling this aggressive species whose huge colonies cause tremendous damage in the United States, Australia, China, and here in Taiwan. To date, although many approaches have been proposed, no method has been successful in stopping the fire ant from spreading. Thus, alternative control methods are urgently needed.

　　Genes can recombine with neighboring genes located on their homologous chromosomes. However, in the fire ant over 600 genes (4% of total) are locked in a non-recombining ‘supergene’ (a large region) by a chromosomal rearrangement. This ‘supergene’ has two variants analogous to the human X and Y sex chromosomes. In the single-queen social form, only one variant is found (like XX females), whereas in the multiple-queen social form both variants are present (like XY males). These ‘social chromosome’ variants result in differences in many queen and worker characteristics. For example, prior to mating, virgin daughter queens from single-queen colonies accumulate abundant fat. When queens start independent nests, this fat is converted into food to feed the initial workers. In contrast, virgin queens from multiple-queen colonies accumulate little fat and adopt a different strategy of joining existing multiple-queen nests. Queens from different social forms also have different scents. Workers in multiple-queen colonies use the scent to accept rm 35-01 cone daughter queens only from multiple-queen colonies. Together, these phenotypic and behavioral interactions between queens and workers, along with other characteristics regulated by these 600 genes, result in the stable single- or multiple-queen societies. In essence, the ‘supergene’ guarantees that the individuals are well adapted to the colony in which they live.

　　This study was conducted with several international collaborators including Prof. Laurent Keller of the University of Lausanne, Switzerland; YannickWurm of  Queen Mary College of London, UK; and DeWayne Shoemaker of the US Department of Agriculture’s Agricultural Research Service.

　　The complete article entitled “A Y-like social chromosome causes alternative colony organization in fire ants” is available as an Advance Online Publication at the Nature website at: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11832.html.

　　The complete list of authors is: John Wang, Yannick Wurm, Mingkwan Nipitwattanaphon, Oksana Riba-Grognuz, Yu-Ching Huang, DeWayne Shoemaker, and Laurent Keller.