Evolution of male pregnancy associated with remodeling of canonical vertebrate immunity in seahorses and pipefishes

Abstract

A fundamental problem for the evolution of pregnancy, the most specialized form of parental investment among vertebrates, is the rejection of the nonself-embryo. Mammals achieve immunological tolerance by down-regulating both major histocompatibility complex pathways (MHC I and II). Although pregnancy has evolved multiple times independently among vertebrates, knowledge of associated immune system adjustments is restricted to mammals. All of them (except monotremata) display full internal pregnancy, making evolutionary reconstructions within the class mammalia meaningless. Here, we study the seahorse and pipefish family (syngnathids) that have evolved male pregnancy across a gradient from external oviparity to internal gestation. We assess how immunological tolerance is achieved by reconstruction of the immune gene repertoire in a comprehensive sample of 12 seahorse and pipefish genomes along the "male pregnancy" gradient together with expression patterns of key immune and pregnancy genes in reproductive tissues. We found that the evolution of pregnancy coincided with a modification of the adaptive immune system. Divergent genomic rearrangements of the MHC II pathway among fully pregnant species were identified in both genera of the syngnathids: The pipefishes (Syngnathus) displayed loss of several genes of the MHC II pathway while seahorses (Hippocampus) featured a highly divergent invariant chain (CD74). Our findings suggest that a trade-off between immunological tolerance and embryo rejection accompanied the evolution of unique male pregnancy. That pipefishes survive in an ocean of microbes without one arm of the adaptive immune defense suggests a high degree of immunological flexibility among vertebrates, which may advance our understanding of immune-deficiency diseases.

Keywords: comparative genomics; immunological tolerance; major histocompatibility complex; male pregnancy; seahorse.

Fig. 1.

Morphology of brood pouches of subfamily Nerophinae (A), and of the genera Syngnathus (B), and Hippocampus (C) and display of the placenta-like structures in syngnathids (only Syngnathus and Hippocampus). The placenta-like structure with lumen, apical pore, CRM (cells rich in mitochondria), ions, epithelial cells, capillaries, and the egg are drawn after figure 1 of ref. .

Fig. 2.

Remodeling and loss of key genes of the MHC class I and II pathways among 14 species of Syngnathiformes. Species are arranged according to their phylogeny (SI Appendix, Fig. S1). The copy numbers of the most important genes of the MHC I pathway (MHC I, B2M, CD8a, CD8b and TAP1, TAP2, TAP2T) and MHC II pathway (MHC IIα, MHC IIβ, AICDA [AID], AIRE, CD4, CD74, CIITA, RAG1, RAG2) are displayed. Superscript “1” specifies large fragmented genomes; superscript “2” specifies previously published genomes. White boxes indicate loss of genes and half green boxes indicate loss of exons.

Fig. 3.

Domain-level alignment of three genes (CD74, AIRE, CIITA) critical for the MHC class II pathway with lost or divergent exons in Hippocampus and Syngnathus compared to other vertebrate sequences. For each exon, sequence divergence to other teleosts (yellow), human (34), or sequence conservation other teleosts (green), human (blue) are shown. Exon loss is indicated in white and sequence conservation between Hippocampus and Syngnathus but divergence to other teleosts/humans in gray. Orange triangles indicate the number of sites under positive selection. An ansterisk (*) indicates yellow exon 6b and 7 of CD74 also diverge between Syngnathus and Hippocampus.

Fig. 4.

Cooption of genes known from female human pregnancy in syngnathid male pregnancy. (A) Human female pregnancy and the most important immunological, developmental, and endocrinological pathways involved. (B) Genes involved in those pathways with an established role in human pregnancy change their expression also during pipefish pregnancy.

Fig. 5.

Heatmap displaying expression changes (log₂ fold-change) of S. typhle immune genes during pouch development (DEV), early pregnancy (EP), and late pregnancy (LP). Homologs of genes marked with (M) possess known function (bold) in mammals (M), designation (R) denotes differential expression in the squamate reptile C. ocellatus (R), in the seahorse H. abdominalis (S), or in the pipefish species S. scovelli (P).