3.1. Identification of BpeAQP Family Members and Subfamily Classification

BpeAQP genes were searched against the sequenced birch genome available on NCBI (https://blast.ncbi.nlm.nih.gov/Blast.cgi; accessed on 2 July 2021) and the CoGe comparative genomics platform (https://genomevolution.org/coge/). The investigations were conducted using keyword queries (“Major Intrinsic Protein” and “Aquaporin”), complemented by tBLASTn searches with conservative criteria requiring a cut-off of E-value of 1.0⁻⁵ by using the amino acid sequences of aquaporin sequences from A. thaliana (AtAQP) [2] and Olea europaea L. (OleaAQP) [5] as queries. The B. pendula AQP sequences (BpeAQP) were named according to the standardized MIP nomenclature, and each name (i.e., PIP, TIP, NIP, SIP, and XIP) was guided by sequence similarity and phylogenetic analysis with orthologs from A. thaliana, O. europaea, Lycopersicon esculentum (SlAQP; [144]) and Populus trichocarpa (PoptrAQP, [14]). Each BpeAQP gene accession is derived from the identifier of the GenBank GSS contig on which it is located. When necessary, it is subscripted with an additional lowercase letter to indicate that BpeAQP genes accessions differing only by this letter were originally assembled in the same GSS contig. The Betula AQP protein names, accession numbers, and sequences (genomic, CDS, and deduced proteins) used in this work are listed in Figure 1, Figure S1 and Table S1, respectively.