Goulden and Jackson (1996) introduced, using Jack symmetric functions, some multivariate generating series ψ(x, y, z; 1, 1 + β) with an additional parameter β that may be interpreted as a continuous deformation of the rooted bipartite maps generating series. Indeed, it has the property that for β ∈ {0, 1}, it specializes to the rooted, orientable (general, i.e. orientable or not, respectively) bipartite maps generating series. They made the following conjecture: coefficients of ψ are polynomials in β with positive integer coefficients that can be written as a multivariate generating series of rooted, general bipartite maps, where the exponent of β is an integer-valued statistic that in some sense "measures the non-orientability" of the corresponding bipartite map.We show that except for two special values of β = 0, 1 for which the combinatorial interpretation of the coefficients of ψ is known, there exists a third special value β = −1 for which the coefficients of ψ indexed by two partitions µ, ν, and one partition with only one part are given by rooted, orientable bipartite maps with arbitrary face degrees and black/white vertex degrees given by µ/ν, respectively. We show that this evaluation corresponds, up to a sign, to a top-degree part of the coefficients of ψ. As a consequence, we introduce a collection of integer-valued statistics of maps (η) such that the top-degree of the multivariate generating series of rooted bipartite maps with only one face (called unicellular) with respect to η gives the top-degree of the appropriate coefficients of ψ. Finally, we show that the b-conjecture holds true for all rooted, unicellular bipartite maps of genus at most 2.