A study by Wang et al. (2013) tells that two chromosome variants in fire ants, called the social B and social b (SB and Sb), are characterized by a large region of approximately 13 megabases in which recombination is completely suppressed between SB and Sb. Recombination occurs normally between SB:s but not between Sb:s because Sb/Sb individuals are non-viable. Genomic comparisons revealed limited differentiation between SB and Sb, and the majority of 616 genes identified in the non-recombining region are present in the both variants. The lack of recombination over more than half of the two heteromorphic social chromosomes can be explained by at least one large inversion of around 9 megabases. So again inversions seem to play a large role in causing recombination suppression and the Sb chromosome shows similarities with sex chromosomes in other systems (X and Y, Z and W) and in particular with U and V of algea and bryophytes.
Figure 2 in Wang et al. showing the fire ant's 16 chromosomes with the social chromosome S and its non recombining part.