<p>Collaboration between mathematicians and mathematics educators is widely recognized as a key strategy for enhancing mathematics teacher education. Prior research has identified instances of disciplinary boundary crossing and identity transformation in these collaborations, primarily in small-scale, research-driven settings. However, little is known about how these processes unfold naturally within routine collaborative environments. This large-scale study addresses this gap by examining Mathematics Teacher Educators' (MTEs) self-identifications and cross-disciplinary research tendencies within hybrid mathematics education departments in Turkey—unique settings that foster sustained interactions shaping identity development and boundary negotiation. Using data from 184 participants collected through the Higher Education Council's database and direct surveys, the study employs descriptive statistics, cross-tabulation, and correlational analyses to investigate relationships among key variables, including PhD fields, academic rank, gender, and school teaching experience. Our findings reveal considerable variability in self-identifications across PhD fields, with notable divergences in disciplinary affiliations. Mathematics PhD holders, in particular, demonstrate professional identities that extend beyond traditional disciplinary boundaries, influenced by factors such as academic rank and school teaching experience. In contrast, MTEs with PhDs in mathematics education exhibit strong alignment with their field and engage in limited cross-disciplinary activity. The study also identifies three patterns of identity transformation: bounded, transitioned, and hybrid professionals. These findings contribute to understanding how hybrid environments shape professional identities and facilitate interdisciplinary engagement among MTEs.</p>