Slugs’ last meals: molecular identification of sequestered chloroplasts from different algal origins in Sacoglossa (Opisthobranchia, Gastropoda)

Abstract

Some sacoglossan sea slugs have become famous for their unique capability to extract and incorporate functional chloroplasts from algal food organisms (mainly Ulvophyceae) into their gut cells. The functional incorporation of the so-called kleptoplasts allows the slugs to rely on photosynthetic products for weeks to months, enabling them to survive long periods of food shortage over most of their life-span. The algal food spectrum providing kleptoplasts as temporary, non-inherited endosymbionts appears to vary among sacoglossan slugs, but detailed knowledge is sketchy or unavailable. Accurate identification of algal donor species, which provide the chloroplasts for long-term retention is of primary importance to elucidate the biochemical mechanisms allowing long-term functionality of the captured chloroplast in the foreign animal cell environment. Whereas some sacoglossans forage on a variety of algal species, (e.g. Elysia crispata and E. viridis) others are more selective. Hence, characterizing the range of functional sacoglossan-chloroplast associations in nature is a prerequisite to understand the basis of this enigmatic endosymbiosis. Here, we present a suitable chloroplast gene (tufA) as a marker, which allows identification of the respective algal kleptoplast donor taxa by analysing DNA from whole animals. This novel approach allows identification of donor algae on genus or even species level, thus providing evidence for the taxonomic range of food organisms. We report molecular evidence that chloroplasts from different algal sources are simultaneously incorporated in some species of Elysia. NeigborNet analyses for species assignments are preferred over tree reconstruction methods because the former allow more reliable statements on species identification via barcoding, or rather visualize alternative allocations not to be seen in the latter.

Phylogenetic evidence for multiple independent origins of functional kleptoplasty in Sacoglossa (Heterobranchia, Gastropoda)

“…This may indicate flaws of the applied marker rbcL in identifying Caulerpa sequences or reflects the fact that hardly any Caulerpa species were found during sample collection. Händeler et al (2010) and Christa et al (2013) identified several Caulerpa species in Elysia tomentosa, Volvatella viridis, and P. ocellatus by applying tufA as barcoding marker, whereas no Caulerpa sequences were obtained in the latter study when applying rbcL. In either way, identified foot items based on DNA barcoding should only be interpreted as the minimum possible consumed algae, but handled with care in drawing overall conclusions on food preferences.…”

Photoprotective Non-photochemical Quenching Does Not Prevent Kleptoplasts From Net Photoinactivation

“…Many Sacoglossa feed on alga that lack important photoprotection mechanisms, such as the xanthophyll cycle (XC) and the q E part of NPQ (Cruz et al, 2015;Christa et al, 2017), and with regard to the kleptoplast longevity, there is no difference between the various retention forms since NR, StR and LtR forms feed on both types of algae and keep them for comparable starvation periods (Händeler et al, 2010;Christa et al, 2014b). The most prominent examples are Elysia chlorotica, which feeds on Vaucheria litorea, an alga that possess both mechanisms (Cruz et al, 2015), and Plakobranchus ocellatus, that consumes only XC and q E -deficient species (Maeda et al, 2012;Christa et al, 2013;Wade and Sherwood, 2018).…”

Inventory of Japanese Sacoglossan Opisthobranchs: Historical Review, Current Records, and Unresolved Issues

“…Sue Williams (2000aWilliams ( , 2000b used PAM and mass spectrometry to investigate the photobiology of Australasian sacoglossans for her Ph.D. research at University of Western Australia at Perth; the methodology was subsequently used by Evertsen et al (2007), , and Händeler et al (2009Händeler et al ( , 2010 for a suite of Indo-Pacifi c and Mediterranean species, several of which also occur in Japan. This methodology was recently employed by Yamamoto et al (2008Yamamoto et al ( , 2009) to investigate Japanese sacoglossans; they found photosynthetically active chlorophyll in ca.…”