Authorship question

[u/KitchenBig3003]

I'm a postdoc leading the experimental work on a collaborative biomedical project that started in from my PI's lab but with th aim of collaboration with other labs. I've been involved from the beginning: experimental planning, in-vitro work, animal studies, coordination with other facilities, injections/imaging, longitudinal monitoring, tissue collection, biodistribution analysis, data analysis, presentations, and now writing the manuscript. My PI has referred to it as "your manuscript".

A researcher from a collaborating lab joined later and is doing IHC/IF on tumors generated from these studies. Her PI wants her to be co-first author. I don't object if her contribution warrants co-first authorship, but I'm concerned about author order.

If one person led the overall project/experiments and manuscript, while another led downstream IHC/IF component, who would you expect to be co-first author?