Modeling variability in product lines (PL) has received a lot of attention in recent years, building on the idea that product could be automatically derived from a PL through model transformations, at leastbfor its static architecture (e.g. class diagrams). This paper proposes to go beyond these static aspects by also addressing the behavioral aspect of software product lines. Inspired by the way UML2.0 sequence diagrams can be algebraically composed, we propose to specify PL behavioral requirements as algebraic expressions extended with constructs to specify variability. Then we propose a two stages approach to synthesize detailed behavior for each product member in the PL. The first stage uses abstract interpretation of the variability operators in scenarios to get behavior specialization of the PL according to a given decision criteria. The second stage uses statechart synthesis from product expressions. We describe the interest of our method on a well known case study, and briefly discusses its implementation in a prototype tool.