The iron-responsive genome of the chiton Acanthopleura granulata

Molluscs biomineralize structures that vary in composition, form, and
function, prompting questions about the genetic mechanisms responsible for
their production and the evolution of these mechanisms. Chitons (Mollusca,
Polyplacophora) are a promising system for studies of biomineralization
because they build a range of calcified structures including shell plates
and spine- or scale-like sclerites. Chitons also harden the calcified
teeth of their rasp-like radula with a coat of iron (as magnetite). Here
we present the genome of the West Indian fuzzy chiton Acanthopleura
granulata, the first from any aculiferan mollusc. The A. granulata genome
contains homologs of many genes associated with biomineralization in
conchiferan molluscs. We expected chitons to lack genes previously
identified from pathways conchiferans use to make biominerals like calcite
and nacre because chitons do not use these materials in their shells.
Surprisingly, the A. granulata genome has homologs of many of these genes,
suggesting that the ancestral mollusc may have had a more diverse
biomineralization toolkit than expected. The A. granulata genome has
features that may be specialized for iron biomineralization, including a
higher proportion of genes regulated directly by iron than other molluscs.
A. granulata also produces two isoforms of soma-like ferritin: one is
regulated by iron and similar in sequence to the soma-like ferritins of
other molluscs, and the other is constitutively translated and is not
found in other molluscs. The A. granulata genome is a resource for future
studies of molluscan evolution and biomineralization.