This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/NPH.17478 This article is protected by copyright. All rights reserved DR TIMOTHY STEPHENS (Orcid ID : 0000-0003-1554-7175) PROF. DEBASHISH BHATTACHARYA (Orcid ID : 0000-0003-0611-1273) Tansley insight Why is primary endosymbiosis so rare? Timothy G. Stephens 1 , Arwa Gabr 2 , Victoria Calatrava 3 , Arthur R. Grossman 3 , Debashish Bhattacharya 1 1 Rutgers University, Department of Biochemistry and Microbiology, New Brunswick, New Jersey 08901, USA 2 Graduate Program in Molecular Bioscience and Program in Microbiology and Molecular Genetics, Rutgers University, Piscataway, New Jersey 08854, USA 3 The Carnegie Institution, Department of Plant Biology, Stanford, California 94305, USA Author for correspondence: Debashish Bhattacharya Tel: +1 848-932-6218 E-mail: d.bhattacharya@rutgers.edu Received: 29 January 2021 Accepted: 5 May 2021 ORCID Timothy Stephens https://orcid.org/0000-0003-1554-7175 Arwa Gabr https://orcid.org/0000-0002-7159-0568 Victoria Calatrava https://orcid.org/0000-0002-6323-5793 Author Manuscript