High variability in complementarity‐determining regions compensates for a low number of V gene families in the λ light chain locus of the platypus

Abstract

Based on the analysis of a panel of variable (V) region sequences from the Australian duck-billed platypus and the Australian short beaked echidna, the monotremes were found to express a highly diversified Vk repertoire. High variability was observed both in sequence and in length of all three CDR regions. However, all monotreme sequences were found to form a separate branch on a distance tree, and the monotremes appear to express only two Vk gene families. The appearance of all Vk gene segments in one branch on the distance tree gives further support for the notion that deletions of entire V region clans or families, followed by successive rounds of gene duplications may be a relatively common phenomenon during vertebrate evolution. Four different constant region sequences were also identified and a preferential use of certain J segments to each constant region was observed. A more detailed picture of the locus was obtained by analysis of genomic DNA by Southern blot and PCR. The organization of the lambda locus involves multiple V and several constant region genes with one or several joining segments positioned upstream of each constant region, similar to the organization in mouse and man. An mRNA frequency analysis shows that the k light chain accounts for more than 90% of the light chain transcripts in the spleen. The abundance and the high variability indicate that light chain diversity at the k locus contributes significantly to the antigen-binding repertoire in monotremes. A high k to j light chain ratio also indicates that variability in the CDR regions is more important for the repertoire size than the total number of V gene families.

Four primordial immunoglobulin light chain isotypes, including λ and κ, identified in the most primitive living jawed vertebrates

“…Common themes in both studies are differential use of the V L with shorter CDR1 and longer CDR2 (r and r-cart) compared to isotypes with the opposite trend in CDR lengths (k and j) either in gut-associated lymphoid tissue or with IgH expressed in gut-associated lymphoid tissue. Interestingly in the monotreme platypus (representative of a more ancestral mammalian group), no Igr has been described but Igk V segments display exceptionally high CDR length diversity, perhaps in compensation for the loss of that diversity from Igr [35].…”

Those other mammals: The immunoglobulins and T cell receptors of marsupials and monotremes

“…All marsupial and monotreme species examined so far have homologues of both Igκ and Igλ [50–55]. Like heavy chains, the genomic organization of the light chain loci in M. domestica is un-remarkable and highly conserved with that of eutherian mammals [33].…”

“…Interestingly, monotremes more closely follow the co-evolutionary rule. The platypus has limited VH diversity and more limited light chain V diversity [54,55,60]. The platypus appears to compensate for the limited germ-line diversity in both heavy and light chains with extensive CDR3 diversity [55,60].…”

Cloning and structural analysis of two highly divergent IgA isotypes, IgA1 and IgA2 from the duck billed platypus, Ornithorhynchus anatinus

“…For additional information on mRNA frequencies from different light chain genes see Johansson et al (2005).…”

“…For a comparative analysis the relative expression levels of the two light chains was also included (Johansson et al, 2005;Nowak et al, 2004). To avoid potential cross-hybridization, cDNA fragments from the con- Fig.…”