Diversification of innate immune genes: lessons from the purple sea urchin

Local Genomic Instability of the SpTransformer Gene Family in the Purple Sea Urchin Inferred from BAC Insert Deletions

The SpTransformer (SpTrf) gene family in the purple sea urchin, Strongylocentrotus purpuratus, encodes immune response proteins. The genes are clustered, surrounded by short tandem repeats, and some are present in genomic segmental duplications. The genes share regions of sequence and include repeats in the coding exon. This complex structure is consistent with putative local genomic instability. Instability of the SpTrf gene cluster was tested by 10 days of growth of Escherichia coli harboring bacterial artificial chromosome (BAC) clones of sea urchin genomic DNA with inserts containing SpTrf genes. After the growth period, the BAC DNA inserts were analyzed for size and SpTrf gene content. Clones with multiple SpTrf genes showed a variety of deletions, including loss of one, most, or all genes from the cluster. Alternatively, a BAC insert with a single SpTrf gene was stable. BAC insert instability is consistent with variations in the gene family composition among sea urchins, the types of SpTrf genes in the family, and a reduction in the gene copy number in single coelomocytes. Based on the sequence variability among SpTrf genes within and among sea urchins, local genomic instability of the family may be important for driving sequence diversity in this gene family that would be of benefit to sea urchins in their arms race with marine microbes.

The lysosome-phagosome pathway mediates immune regulatory mechanisms in Mesocentrotus nudus against Vibrio coralliilyticus infection

Sea urchins are a popular model species for studying invertebrate diseases. The immune regulatory mechanisms of the sea urchin Mesocentrotus nudus during pathogenic infection are currently unknown. This study aimed to reveal the potential molecular mechanisms of M. nudus during resistance to Vibrio coralliilyticus infection by integrative transcriptomic and proteomic analyses. Here, we identified a total of 135,868 unigenes and 4,351 proteins in the four infection periods of 0 h, 20 h, 60 h and 100 h in M. nudus. In the I20, I60 and I100 infection comparison groups, 10,861, 15,201 and 8,809 differentially expressed genes (DEGs) and 2,188, 2,386 and 2,516 differentially expressed proteins (DEPs) were identified, respectively. We performed an integrated comparative analysis of the transcriptome and proteome throughout the infection phase and found very a low correlation between transcriptome and proteome changes. KEGG pathway analysis revealed that most upregulated DEGs and DEPs were involved in immune strategies. Notably, "lysosome" and "phagosome" activated throughout the infection process, could be considered the two most important enrichment pathways at the mRNA and protein levels. The significant increase in phagocytosis of infected M. nudus coelomocytes further demonstrated that the lysosome-phagosome pathway played an important immunological role in M. nudus resistance to pathogenic infection. Key gene expression profiles and protein‒protein interaction analysis revealed that cathepsin family and V-ATPase family genes might be key bridges in the lysosome-phagosome pathway. In addition, the expression patterns of key immune genes were verified using qRT‒PCR, and the different expression trends of candidate genes reflected, to some extent, the regulatory mechanism of immune homeostasis mediated by the lysosome-phagosome pathway in M. nudus against pathogenic infection. This work will provide new insights into the immune regulatory mechanisms of sea urchins under pathogenic stress and help identify key potential genes/proteins for sea urchin immune responses.

Response to microplastic exposure: An exploration into the sea urchin immune cell proteome

It is now known that the Mediterranean Sea currently is one of the major hotspot for microplastics (MPs; < 5 mm) pollution and that the risks will be even more pronounced in the coming years. Thus, the in-depth study of the mechanisms underlying the MPs toxicity in key Mediterranean organisms, subjected to high anthropic pressures, has become a categorical imperative to pursue. Here, we explore for the first time the sea urchins immune cells profile combined to their proteome upon in vivo exposure (72 h) to different concentrations of polystyrene-microbeads (micro-PS) starting from relevant environmental concentrations (10, 50, 10³, 10⁴ MP/L). Every 24 h, immunological parameters were monitored. After 72 h, the abundance of MPs was examined in various organs and coelomocytes were collected for proteomic analysis based on a shotgun label free proteomic approach. While sea urchins treated with the lowest concentration tested (10 and 50 micro-PS/L) did not show the presence of micro-PS in any tissue, in the specimens exposed to the highest concentration (10³ and 10⁴ micro-PS) there was an internalisation of 9.75 ± 2.75 and 113.75 ± 34.5 MP/g, respectively. Proteomic analyses revealed that MPs exposure altered coelomocytes protein profile not only compared to the control group but also among the different micro-PS concentrations and these variations are micro-PS concentration dependent. The proteins exclusively expressed in the coelomocytes of specimens exposed to MPs are mainly metabolite interconversion enzymes, involved in cellular processes, indicating a severe alteration of the cellular metabolic pathways. Overall, these findings provide new insights on the mode of action of MPs in the sea urchin immune cells both at the molecular and cellular level.

Sequence Diversity, Locus Structure, and Evolutionary History of the SpTransformer Genes in the Sea Urchin Genome

The generation of large immune gene families is often driven by evolutionary pressure exerted on host genomes by their pathogens, which has been described as the immunological arms race. The SpTransformer (SpTrf) gene family from the California purple sea urchin, Strongylocentrotus purpuratus, is upregulated upon immune challenge and encodes the SpTrf proteins that interact with pathogens during an immune response. Native SpTrf proteins bind both bacteria and yeast, and augment phagocytosis of a marine Vibrio, while a recombinant SpTrf protein (rSpTrf-E1) binds a subset of pathogens and a range of pathogen associated molecular patterns. In the sequenced sea urchin genome, there are four SpTrf gene clusters for a total of 17 genes. Here, we report an in-depth analysis of these genes to understand the sequence complexities of this family, its genomic structure, and to derive a putative evolutionary history for the formation of the gene clusters. We report a detailed characterization of gene structure including the intron type and UTRs with conserved transcriptional start sites, the start codon and multiple stop codons, and locations of polyadenylation signals. Phylogenetic and percent mismatch analyses of the genes and the intergenic regions allowed us to predict the last common ancestral SpTrf gene and a theoretical evolutionary history of the gene family. The appearance of the gene clusters from the theoretical ancestral gene may have been driven by multiple duplication and deletion events of regions containing SpTrf genes. Duplications and ectopic insertion events, indels, and point mutations in the exons likely resulted in the extant genes and family structure. This theoretical evolutionary history is consistent with the involvement of these genes in the arms race in responses to pathogens and suggests that the diversification of these genes and their encoded proteins have been selected for based on the survival benefits of pathogen binding and host protection.

Molecular characterization and expression patterns of two LPS binding /bactericidal permeability-increasing proteins (LBP/BPIs) from the scallop Argopecten purpuratus

Lipopolysaccharide-binding proteins (LBPs) and bactericidal permeability-increasing proteins (BPIs) are effectors of the innate immune response which act in a coordinated manner to bind and neutralize the LPS present in Gram negative bacteria. The structural organization that confers the function of LBPs and BPIs is very similar, however, they are antagonistic to each other. In this work, we characterized two LBP/BPIs from the scallop Argopecten purpuratus, namely ApLBP/BPI1 and ApLBP/BPI2. The molecular and phylogenetic analyses of ApLBP/BPIs indicated that both isoforms display classic characteristics of LBP/BPIs from other invertebrates. Additionally, ApLBP/BPIs are constitutively expressed in scallop tissues and their transcript expression is upregulated in hemocytes and gills in response to an immune challenge. However, some structural characteristics of functional importance for the biological activity of these molecules, such as the net charge differ substantially between ApLBP/BPI1 and ApLBP/BPI2. Furthermore, each isoform displays a specific profile of basal expression among different tissues, as well as specific patterns of expression during the activation of the immune response. Results suggest that functional specialization of ApLBP/BPIs might happen, with potential role as LBP or BPI in this species of scallop. Further research on the biological activities of ApLBP/BPIs are necessary to elucidate their participation in the scallop immune response.

Transcriptome profiling reveals key roles of phagosome and NOD-like receptor pathway in spotting diseased Strongylocentrotus intermedius

Spotting disease is a common disease in the process of aquaculture and restocking of the sea urchin Strongylocentrotus intermedius and leads to mass mortality. To characterize the molecular processes and candidate genes related to spotting disease in S. intermedius, we conducted next-generation sequencing to assess the key genes/pathways in spotting diseased sea urchin (DUG) compared to healthy ones (HUG). A total of 321.1 million clean reads were obtained and assembled into 93,877 Unigenes with an N50 of 1185 bp, in which 86.48% of them matched to the genome sequence of the sea urchin S. purpuratus and 27,456 Unigenes mapped to Nr database. Salmon expression analysis revealed 1557 significantly differently expressed genes (DEGs) between DUG and HUG. These DEGs were enriched into 151 KEGG pathways including a core set of immune correlated pathways notably in phagosome and NOD-like receptor signaling. DUG displayed an obvious downregulation in these immune pathways. The expression patterns of six DEGs were confirmed by RT-qPCR, and the expressions were consistent with the results of RNA-seq. Furthermore, 15,990 SSRs were identified and a total of 235,249 and 295,567 candidate SNPs were identified from DUG and HUG, respectively. All these results provided basic information for our understanding of spotting disease outbreak in sea urchin.

Short tandem repeats, segmental duplications, gene deletion, and genomic instability in a rapidly diversified immune gene family

BACKGROUND

Genomic regions with repetitive sequences are considered unstable and prone to swift DNA diversification processes. A highly diverse immune gene family of the sea urchin (Strongylocentrotus purpuratus), called Sp185/333, is composed of clustered genes with similar sequence as well as several types of repeats ranging in size from short tandem repeats (STRs) to large segmental duplications. This repetitive structure may have been the basis for the incorrect assembly of this gene family in the sea urchin genome sequence. Consequently, we have resolved the structure of the family and profiled the members by sequencing selected BAC clones using Illumina and PacBio approaches.

RESULTS

BAC insert assemblies identified 15 predicted genes that are organized into three clusters. Two of the gene clusters have almost identical flanking regions, suggesting that they may be non-matching allelic clusters residing at the same genomic locus. GA STRs surround all genes and appear in large stretches at locations of putatively deleted genes. GAT STRs are positioned at the edges of segmental duplications that include a subset of the genes. The unique locations of the STRs suggest their involvement in gene deletions and segmental duplications. Genomic profiling of the Sp185/333 gene diversity in 10 sea urchins shows that no gene repertoires are shared among individuals indicating a very high gene diversification rate for this family.

CONCLUSIONS

The repetitive genomic structure of the Sp185/333 family that includes STRs in strategic locations may serve as platform for a controlled mechanism which regulates the processes of gene recombination, gene conversion, duplication and deletion. The outcome is genomic instability and allelic mismatches, which may further drive the swift diversification of the Sp185/333 gene family that may improve the immune fitness of the species.

De novo assembly and analysis of tissue-specific transcriptomes revealed the tissue-specific genes and profile of immunity from Strongylocentrotus intermedius

Strongylocentrotus intermedius is an important marine species in north China and Japan. Recent years, diseases are threating the sea urchin aquaculture industry seriously. To provide a genetic resource for S. intermedius as well as overview the immune-related genes of S. intermedius, we performed transcriptome sequencing of three cDNA libraries representing three tissues, coelomocytes, gut and peristomial membrane respectively. In total 138,421 contigs were assembled from all sequencing data. 96,764 contigs were annotated according to bioinformatics databases, including NT, nr, Swiss-Prot, KEGG, COG. 49,336 Contigs were annotated as CDS. In this study, we obtained 24,778 gene families from S. intermedius transcriptome. The gene expression analysis revealed that more genes were expressed in gut, more high expression level genes in coelomocytes when compared with other tissues. Specific expressed contigs in coelomocytes, gut, and peristomial membrane were 546, 1136, and 1012 respectively. Pathway analysis suggested 25, 17 and 36 potential specifically pathways may specific progressed in peristomial membrane, gut and coelomocytes respectively. Similarities and differences between S. intermedius and other echinoderms were analyzed. S. intermedius was more homology to Strongylocentrotus purpuratus than others sea urchin. Of 24,778 genes, 1074 genes are immune-related, immune genes were expressed with a higher level in coelomocytes than other tissues. Complement system may be the most important immune system in sea urchin. We also identified 2438 SSRs and 16,236 SNPs for S. intermedius. These results provide a transcriptome resource and foundation to study molecular mechanisms of sea urchin immune system.

Titanium dioxide nanoparticles stimulate sea urchin immune cell phagocytic activity involving TLR/p38 MAPK-mediated signalling pathway

Titanium dioxide nanoparticles (TiO2NPs) are one of the most widespread-engineered particles in use for drug delivery, cosmetics, and electronics. However, TiO2NP safety is still an open issue, even for ethical reasons. In this work, we investigated the sea urchin Paracentrotus lividus immune cell model as a proxy to humans, to elucidate a potential pathway that can be involved in the persistent TiO2NP-immune cell interaction in vivo. Morphology, phagocytic ability, changes in activation/inactivation of a few mitogen-activated protein kinases (p38 MAPK, ERK), variations of other key proteins triggering immune response (Toll-like receptor 4-like, Heat shock protein 70, Interleukin-6) and modifications in the expression of related immune response genes were investigated. Our findings indicate that TiO2NPs influence the signal transduction downstream targets of p38 MAPK without eliciting an inflammatory response or other harmful effects on biological functions. We strongly recommend sea urchin immune cells as a new powerful model for nano-safety/nano-toxicity investigations without the ethical normative issue.

Extraordinary Diversity of Immune Response Proteins among Sea Urchins: Nickel-Isolated Sp185/333 Proteins Show Broad Variations in Size and Charge

Effective protection against pathogens requires the host to produce a wide range of immune effector proteins. The Sp185/333 gene family, which is expressed by the California purple sea urchin Strongylocentrotus purpuratus in response to bacterial infection, encodes a highly diverse repertoire of anti-pathogen proteins. A subset of these proteins can be isolated by affinity to metal ions based on multiple histidines, resulting in one to four bands of unique molecular weight on standard Western blots, which vary depending on the individual sea urchin. Two dimensional gel electrophoresis (2DE) of nickel-isolated protein samples followed by Western blot was employed to detect nickel-isolated Sp185/333 (Ni-Sp185/333) proteins and to evaluate protein diversity in animals before and after immune challenge with marine bacteria. Ni-Sp185/333 proteins of the same molecular weight on standard Western blots appear as a broad complex of variants that differ in pI on 2DE Western blots. The Ni-Sp185/333 protein repertoire is variable among animals, and shows a variety of changes among individual sea urchins in response to immune challenges with both the same and different species of bacteria. The extraordinary diversity of the Ni-Sp185/333 proteins may provide significant anti-pathogen capabilities for sea urchins that survive solely on innate immunity.

Up in Arms: Immune and Nervous System Response to Sea Star Wasting Disease

Echinoderms, positioned taxonomically at the base of deuterostomes, provide an important system for the study of the evolution of the immune system. However, there is little known about the cellular components and genes associated with echinoderm immunity. The 2013-2014 sea star wasting disease outbreak is an emergent, rapidly spreading disease, which has led to large population declines of asteroids in the North American Pacific. While evidence suggests that the signs of this disease, twisting arms and lesions, may be attributed to a viral infection, the host response to infection is still poorly understood. In order to examine transcriptional responses of the sea star Pycnopodia helianthoides to sea star wasting disease, we injected a viral sized fraction (0.2 μm) homogenate prepared from symptomatic P. helianthoides into apparently healthy stars. Nine days following injection, when all stars were displaying signs of the disease, specimens were sacrificed and coelomocytes were extracted for RNA-seq analyses. A number of immune genes, including those involved in Toll signaling pathways, complement cascade, melanization response, and arachidonic acid metabolism, were differentially expressed. Furthermore, genes involved in nervous system processes and tissue remodeling were also differentially expressed, pointing to transcriptional changes underlying the signs of sea star wasting disease. The genomic resources presented here not only increase understanding of host response to sea star wasting disease, but also provide greater insight into the mechanisms underlying immune function in echinoderms.

Sea urchin immune cells as sentinels of environmental stress

Echinoderms, an ancient and very successful phylum of marine invertebrates, play a central role in the maintenance of ecosystem integrity and are constantly exposed to environmental pressure, including: predation, changes in temperature and pH, hypoxia, pathogens, UV radiation, metals, toxicants, and emerging pollutants like nanomaterials. The annotation of the sea urchin genome, so closely related to humans and other vertebrate genomes, revealed an unusually complex immune system, which may be the basis for why sea urchins can adapt to different marine environments and survive even in hazardous conditions. In this review, we give a brief overview of the morphological features and recognized functions of echinoderm immune cells with a focus on studies correlating stress and immunity in the sea urchin. Immune cells from adult Paracentrotus lividus, which have been introduced in the last fifteen years as sentinels of environmental stress, are valid tools to uncover basic molecular and regulatory mechanisms of immune responses, supporting their use in immunological research. Here we summarize laboratory and field studies that reveal the amenability of sea urchin immune cells for toxicological testing.

The transcriptome of the NZ endemic sea urchin Kina (Evechinus chloroticus)

Background

Sea urchins are studied as model organisms for developmental and systems biology and also produce highly valued food products. Evechinus chloroticus (Kina) is a sea urchin species that is indigenous to New Zealand. It is the type member of the Evechinus genus based on its morphological characteristics. Previous research has focused on identifying physical factors affecting commercial roe quality of E. chloroticus, but there is almost no genetic information available for E. chloroticus. E. chloroticus is the only species in its genus and has yet to be subject to molecular phylogenetic analysis.

Results

In this study we performed a de novo transcriptome assembly of Illumina sequencing data. A total of 123 million 100 base length paired-end reads were generated using RNA-Seq libraries from a range of E. chloroticus tissues from two individuals obtained from Fiordland, New Zealand. The assembly resulted in a set of 75,002 transcripts with an accepted read coverage and length, of which 24,655 transcripts could be functionally annotated using protein similarity. Transcripts could be further annotated with Gene Ontology, KEGG Orthology and InterPro terms. With this sequence data we could perform the first phylogenetic analysis of E. chloroticus to other species of its family using multiple genes. When sequences for the mitochondrial nitrogen dehydrogenase genes were compared, E. chloroticus remained outside of a family level clade, which indicated E. chloroticus is indeed a genetically distinct genus within its family.

Conclusions

This study has produced a large set of E. chloroticus transcripts/proteins along with functional annotations, vastly increasing the amount of genomic data available for this species. This provides a resource for current and future studies on E. chloroticus, either to increase its commercial value, or its use as a model organism. The phylogenetic results provide a basis for further analysis of relationships between E. chloroticus, its family members, and its evolutionary history.

The Sp185/333 immune response genes and proteins are expressed in cells dispersed within all major organs of the adult purple sea urchin

Purple sea urchins (Strongylocentrotus purpuratus) express a highly variable set of immune genes called Sp185/333 by two subtypes of coelomocytes: the polygonal and small phagocytes. We report that the Sp185/333 genes and their encoded proteins are also expressed in all of the major organs in the adult sea urchin, including the axial organ, pharynx, esophagus, intestine and gonads. After immune challenge, there is an increase in the level of Sp185/333 mRNA in cells associated with the intestine and axial organ. The Sp185/333 proteins increase in the axial organ, pharynx, esophagus and intestine after challenge. However, the proportion of Sp185/333-positive cells only increases in the axial organ, while there is no change in that proportion in the other organs after challenge. The size range of the major Sp185/333 proteins expressed by organs is broader (5 kDa to > 250 kDa) compared with those in coelomocytes (∼40 kDa to < 250 kDa). Images of the different organs do not clarify whether coelomocytes or parenchymal cells express the Sp185/333 proteins. The increase in levels of Sp185/333 transcripts, protein expression and Sp185/333-positive cells in the axial organ in response to challenge suggests that this organ may have an important role in immunity for this species.

Transcriptome sequencing and characterization for the sea cucumber Apostichopus japonicus (Selenka, 1867)

Background

Sea cucumbers are a special group of marine invertebrates. They occupy a taxonomic position that is believed to be important for understanding the origin and evolution of deuterostomes. Some of them such as Apostichopus japonicus represent commercially important aquaculture species in Asian countries. Many efforts have been devoted to increasing the number of expressed sequence tags (ESTs) for A. japonicus, but a comprehensive characterization of its transcriptome remains lacking. Here, we performed the large-scale transcriptome profiling and characterization by pyrosequencing diverse cDNA libraries from A. japonicus.

Results

In total, 1,061,078 reads were obtained by 454 sequencing of eight cDNA libraries representing different developmental stages and adult tissues in A. japonicus. These reads were assembled into 29,666 isotigs, which were further clustered into 21,071 isogroups. Nearly 40% of the isogroups showed significant matches to known proteins based on sequence similarity. Gene ontology (GO) and KEGG pathway analyses recovered diverse biological functions and processes. Candidate genes that were potentially involved in aestivation were identified. Transcriptome comparison with the sea urchin Strongylocentrotus purpuratus revealed similar patterns of GO term representation. In addition, 4,882 putative orthologous genes were identified, of which 202 were not present in the non-echinoderm organisms. More than 700 simple sequence repeats (SSRs) and 54,000 single nucleotide polymorphisms (SNPs) were detected in the A. japonicus transcriptome.

Conclusion

Pyrosequencing was proven to be efficient in rapidly identifying a large set of genes for the sea cucumber A. japonicus. Through the large-scale transcriptome sequencing as well as public EST data integration, we performed a comprehensive characterization of the A. japonicus transcriptome and identified candidate aestivation-related genes. A large number of potential genetic markers were also identified from the A. japonicus transcriptome. This transcriptome resource would lay an important foundation for future genetic or genomic studies on this species.

Massively parallel amplicon sequencing reveals isotype-specific variability of antimicrobial peptide transcripts in Mytilus galloprovincialis

Background

Effective innate responses against potential pathogens are essential in the living world and possibly contributed to the evolutionary success of invertebrates. Taken together, antimicrobial peptide (AMP) precursors of defensin, mytilin, myticin and mytimycin can represent about 40% of the hemocyte transcriptome in mussels injected with viral-like and bacterial preparations, and unique profiles of myticin C variants are expressed in single mussels. Based on amplicon pyrosequencing, we have ascertained and compared the natural and Vibrio-induced diversity of AMP transcripts in mussel hemocytes from three European regions.

Methodology/Principal Findings

Hemolymph was collected from mussels farmed in the coastal regions of Palavas (France), Vigo (Spain) and Venice (Italy). To represent the AMP families known in M. galloprovincialis, nine transcript sequences have been selected, amplified from hemocyte RNA and subjected to pyrosequencing. Hemolymph from farmed (offshore) and wild (lagoon) Venice mussels, both injected with 10⁷Vibrio cells, were similarly processed. Amplicon pyrosequencing emphasized the AMP transcript diversity, with Single Nucleotide Changes (SNC) minimal for mytilin B/C and maximal for arthropod-like defensin and myticin C. Ratio of non-synonymous vs. synonymous changes also greatly differed between AMP isotypes. Overall, each amplicon revealed similar levels of nucleotidic variation across geographical regions, with two main sequence patterns confirmed for mytimycin and no substantial changes after immunostimulation.

Conclusions/Significance

Barcoding and bidirectional pyrosequencing allowed us to map and compare the transcript diversity of known mussel AMPs. Though most of the genuine cds variation was common to the analyzed samples we could estimate from 9 to 106 peptide variants in hemolymph pools representing 100 mussels, depending on the AMP isoform and sampling site. In this study, no prevailing SNC patterns related to geographical origin or Vibrio injection emerged. Whether or not the contact with potential pathogens can increase the amount of AMP transcript variants in mussels requires additional study.

Invertebrate immune diversity

The arms race between hosts and pathogens (and other non-self) drives the molecular diversification of immune response genes in the host. Over long periods of evolutionary time, many different defense strategies have been employed by a wide variety of invertebrates. We review here penaeidins and crustins in crustaceans, the allorecognition system encoded by fuhc, fester and Uncle fester in a colonial tunicate, Dscam and PGRPs in arthropods, FREPs in snails, VCBPs in protochordates, and the Sp185/333 system in the purple sea urchin. Comparisons among immune systems, including those reviewed here have not identified an immune specific regulatory "genetic toolkit", however, repeatedly identified sequences (or "building materials" on which the tools act) are present in a broad range of immune systems. These include a Toll/TLR system, a primitive complement system, an LPS binding protein, and a RAG core/Transib element. Repeatedly identified domains and motifs that function in immune proteins include NACHT, LRR, Ig, death, TIR, lectin domains, and a thioester motif. In addition, there are repeatedly identified mechanisms (or "construction methods") that generate sequence diversity in genes with immune function. These include genomic instability, duplications and/or deletions of sequences and the generation of clusters of similar genes or exons that appear as families, gene recombination, gene conversion, retrotransposition, alternative splicing, multiple alleles for single copy genes, and RNA editing. These commonly employed "materials and methods" for building and maintaining an effective immune system that might have been part of that ancestral system appear now as a fragmented and likely incomplete set, likely due to the rapid evolutionary change (or loss) of host genes that are under pressure to keep pace with pathogen diversity.