Tetrapioid induction of Crassostrea hongkongensis and C.sikamea by inhibiting the polar body 1 release in diploid fertilized eggs

The production of an all-triploid population by mating tetrapioid males with diploid females is the best and most fundamental method for the large-scale production of triploid oysters.Obtaining a stable tetrapioid population is essential for guaranteed production in industrialized triploid cultivation.C.hongkongensis and C.sikamea are important oyster breeding species in southern China,and have great economic value.However,there are not any published data on inducing tetrapioid C.hongkongensis or C.sikamea.Therefore,we investigated tetrapioid induction in these two oyster species by inhibiting the PB1 release in diploid fertilized eggs using Cytochalasin B(CB)under 31℃,15%o salinity.The results confirmed that the optimal tetrapioid induction conditions for C.hongkongensis were a CB concentration of 0.50 mg/L with induction starting at 9.0 min after fertilization,and stopping at 21.0 min after fertilization;the induction efficiency index reached 0.123 under these conditions.The optimal tetrapioid induction conditions for C.sikamea were a CB concentration of 0.50 mg/L,with induction starting at 7.5 min after fertilization and stopping at 18 min after fertilization;the induction efficiency index could be as high as 0.281 under these conditions.However,we confirmed that the tetrapioid rate decreased with larval growth,and no tetrapioids were detected in the juvenile period of either C.hongkongensis or C.sikamea.This may be attributed to the very low survival of the tetrapioid larvae induced by this method,especially as most tetrapioid larvae died during the first three days.In summary,it is simple to directly induce tetrapioid C.hongkongensis and C.sikamea larvae by inhibiting the PB1 release of diploid zygotes,but the low survival rate makes it challenging to obtain viable juvenile tetrapioids.

Adaptive Bird-like Genome Miniaturization During the Evolution of Scallop Swimming Lifestyle

Genome miniaturization drives key evolutionary innovations of adaptive traits in vertebrates,such as the flight evolution of birds.However,whether similar evolutionary processes exist in invertebrates remains poorly understood.Derived from the second-largest animal phylum,scallops are a special group of bivalve molluscs and acquire the evolutionary novelty of the swimming lifestyle,providing excellent models for investigating the coordinated genome and lifestyle evolution.Here,we show for the first time that genome sizes of scallops exhibit a generally negative correlation with locomotion activity.To elucidate the co-evolution of genome size and swimming lifestyle,we focus on the Asian moon scallop(Amusium pleuronectes)that possesses the smallest known scallop genome while being among scallops with the highest swimming activity.Whole-genome sequencing of A.pleuronectes reveals highly conserved chromosomal macrosynteny and microsynteny,suggestive of a highly contracted but not degenerated genome.Genome reduction of A.pleuronectes is facilitated by significant inactivation of transposable elements,leading to reduced gene length,elevated expression of genes involved in energy-producing pathways,and decreased copy numbers and expression levels of biomineralization-related genes.Similar evolutionary changes of relevant pathways are also observed for bird genome reduction with flight evolution.The striking mimicry of genome miniaturization underlying the evolution of bird flight and scallop swimming unveils the potentially common,pivotal role of genome size fluctuation in the evolution of novel lifestyles in the animal kingdom.

Comparative Genomics Reveals Evolutionary Drivers of Sessile Life and Left-right Shell Asymmetry in Bivalves

Bivalves are species-rich mollusks with prominent protective roles in coastal ecosystems.Across these ancient lineages,colony-founding larvae anchor themselves either by byssus production or by cemented attachment.The latter mode of sessile life is strongly molded by left-right shell asymmetry during larval development of Ostreoida oysters such as Crassostrea hongkongensis.Here,we sequenced the genome of C.hongkongensis in high resolution and compared it to reference bivalve genomes to unveil genomic determinants driving cemented attachment and shell asymmetry.Importantly,loss of the homeobox gene Antennapedia(Antp)and broad expansion of lineagespecific extracellular gene families are implicated in a shift from byssal to cemented attachment in bivalves.Comparative transcriptomic analysis shows a conspicuous divergence between leftright asymmetrical C.hongkongensis and symmetrical Pinctada fucata in their expression profiles.Especially,a couple of orthologous transcription factor genes and lineage-specific shell-related gene families including that encoding tyrosinases are elevated,and may cooperatively govern asymmetrical shell formation in Ostreoida oysters.