Consider a multiuser communication system in a frequency selective environment whereby users share a common spectrum and can interfere with each other. Assuming Gaussian signaling and treating interference as noise, we study optimal spectrum sharing strategies for the maximization of weighted sum-rate. In this work, we show that, if the normalized crosstalk gains are larger than a given threshold (roughly equal to 1/2), then the optimal spectrum sharing strategy is Frequency Division Multiple Access (FDMA). We also propose several simple distributed spectrum allocation algorithms that can approximately maximize weighted sum-rates. Numerical simulation of DSL applications shows that these algorithms are efficient and can achieve substantially larger weighted sumrates than those obtained by the existing Iterative Waterfilling algorithm.