RESULTS AND DISCUSSION
The mitogenome of Runcina aurata spanned a total length of
14,282 bp. It was found to contain the typical 13 coding genes (CDS),
the large and small ribosomal RNA genes, and 22 tRNA genes, all of them
with the standard gene order of heterobranchs (Fig. 2). The overall GC%
content was 25.7%, one of the lowest values in heterobranchs. When
focusing on CDS, rRNA and tRNA this value shifted to 26.3%, 23.6% and
25.5% respectively. Parallelly, Facelina auriculatamitochondrial genome length was 14.171, comprising the ordinary 13
coding genes, 2 ribosomal RNA and 22 tRNA, with the standard gene order
of heterobranchs (Fig. 2). The overall GC% content of the mitogenome
was 35%, in contraposition to the R. aurata ’s one. The CDS ofF. auriculata mitogenome were composed of 35.5% GC, the rRNA
were 32% GC and the tRNA were 35.8% GC.
The phylogenetic analysis indicated, as previously reported
(Varney et al., 2021), thatRuncina aurata and therefore Runcinida is sibling taxon to a
clade composed by Aplysiida and Cephalaspidea with a BS support value of
100 (Fig. 3). Following, Facelina auriculata is sibling to the
other facelinid species Sakuraeolis japonica (Baba, 1937), with a
bootstrap support value of 100, being facelinids sibling taxon to the
other aeolidiaceans Protaeollidiella atra Baba, 1955 andHermissenda emurai (Baba, 1937) (BS = 100) (Fig. 3)
(Carmona et al., 2013). With
the R. aurata and F. auriculata mitochondrial reference
genomes, the inner systematic relationships can be properly assessed and
explored, while providing a valuable tool to design specific primers for
mitochondrial targeted markers of these groups, which have been hampered
when using the standard Folmer set for cytochrome oxidase I, and Palumbi
set for 16S.