The chemoreceptor repertoire of D. silvatica
The chemosensory repertoire of chelicerates, particularly of spiders, is
characterized by a high diversity in terms of copy number and sequence
divergence, likely resulted from a constant and prolonged gene birth and
death process, only altered by episodic bursts of gene duplication
yielding lineage-specific expansions (Vizueta et al., 2018). Here, our
comprehensive analysis using the BITACORA pipeline allowed the
identification in the genome of D. silvatica of 545 chemoreceptor
genes, corresponding to 134 Gr and 411 Ir (plus one
homolog to the insect Ir25a and 24 iGluR ) sequences
(Figure 4; Tables 3, S4 and S5). Although these family sizes are in
agreement with the variability observed across spider genomes, the 411Ir reported here represents the largest repertoire found for this
family in chelicerates, only surpassed by the extraordinary
chemoreceptor repertoire found in the German cockroach genome
(Robertson, Baits, Walden, Wada-Katsumata, & Schal, 2018). On the other
hand, the number of Grs in D. silvatica is relatively low
compared to other spiders (i.e. S MIN of 634 Grs
in Parasteatoda tepidariorum ), which could suggest a different
contribution of the gustatory repertories to the evolutionary, and
probably adaptive history of these different spider lineages.
Noticeably, most identified chemoreceptor genes are complete (459
complete genes under the criterion considered in this article; see
methods), yielding a S MIN of 540 genes (Table 3).
This large proportion of complete copies is highly unusual in reports on
these chemoreceptor families across arthropod genomes (Vizueta et al.,
2018), clearly demonstrating the benefits of having a chromosome-level
assembly to annotate, characterize and study multigene families in
animal genomes. The new high-quality assembly has uncovered, for
example, that the two studied gene families are unevenly distributed
across pseudochromosomes. The X chromosome, representing the 23.3% of
the genome assembly, harbors only 3.0% and 3.2% of the Gr andIr identified copies, respectively (Table 3); the members of
these families, nevertheless, are more uniformly distributed across the
other major pseudochromosomes. Even so, the number of chemoreceptors
located in the small scaffolds, which only represent 11.8% of the total
assembly, represents 23.1% and 59.6% of the Gr and Irrepertories, respectively. This uneven distribution, however, is not
observed when considering all genes identified in the genome (Table 3).
While the first feature likely hides some (unknown) particularities of
the sex chromosome, the high representation of gene family members
across the minor scaffolds could be explained by the assembly
difficulties of these repetitive regions. Further studies including gene
families with different family sizes will get insights into this genomic
feature.