Ohta N, Kaplan N, Ng JT, Gravez BJ, Christiaen L.

G3 (Bethesda). 2020 Aug 5;10(8):2697-2711. doi:10.1534/g3.120.401427

Asymmetric Fitness of Second-Generation Interspecific Hybrids Between Ciona robusta and Ciona intestinalis.

Comprehensive introduction on intestinalis-robusta taxonomic situation. Conclusions weakened by the the absence of homotypic F2 C. intestinalis animals caused by the failure of C. intestinalis to reproduce in the culture system. The viability and fertility hybrids born from intestinalis oocytes or robusta (sperm) x intestinalis (oocytes) oocytes is reduced.

“Wild-type Ciona robusta (C. intestinalis type A) and Ciona intestinalis (C. intestinalis type adults were collected in San Diego (CA) and Woods Hole (MA)”. “Sea water (Bio-Actif Salt, Tropic Marin) was controlled by bio-balls (Biomate, Lifegard Aquatics) seeded with bacteria (BioDigest, Prodibio)”. “We obtained hundreds of swimming larvae from each cross [..] this contrasts with previous studies, which suggested that C. robusta oocytes were largely refractory to fertilization by C. intestinalis sperm”. “there were no significant differences in the survival rate between F1 RxI and IxR hybrids”. “By 50 dpf, half of the C. robusta individuals were producing sperm, whereas that proportion dropped significantly for the other groups of animals.”

”For both RxI and IxR hybrids, the majority of animals had [orange pigment organ] at the tip of the sperm duct, in agreement with a previous report (Sato et al. 2014), thus indicating that [orange pigment organ] formation is a dominant trait.”

”C. intestinalis has yellow and orange pigmentation around the tip of siphons that is lacking in C. robusta [...] the majority of RxI and IxR hybrids displayed a bright red pigmentation at the rim of oral and atrial siphons, also consistent with a previous report (Sato et al. 2014). The observation that siphon pigmentation displays an overdominant phenotype in hybrids is consistent with its lack of reliability for taxonomic purposes.“

“the sperm of F1 RxI hybrid appeared less potent to fertilize C. robusta eggs than that of F1 IxR hybrids, which is reminiscent of previously reported difficulties in using C. robusta eggs in interspecific fertilizations.” “Both BC1 (RxI)xR and (IxR)xR hybrids had lower survival rates than F2 C. robusta animals, while an ANOVA did not show significant differences in survival rate on 28 and 50 dpf between (RxI)xR and (IxR)xR hybrids.”

“We obtained sperm from 7 and 10 individuals, and eggs from 7 and 11 F1 RxI and IxR mature animals, respectively, and used them for within-type fertilizations. Fertilization rates were significantly higher for IxR hybrids than for RxI hybrids”

“Finally, F2 IxR hybrids grew and matured to produce sperm and eggs (Table 5 and Supplemental table S3). The sperm and eggs could fertilize each other to produce F3 IxR hybrids, which survived at least 28 dpf, after which we stopped observations.”

“simple quantitative traits, such as body size, showed an increased variability in F2 hybrids as expected for polygenic traits following allele segregation.”