Introduction
Maternal effects can have a large impact on the fitness of offspring. In
plants, maternal effects in seed traits (e.g., seed mass, germination
time) and offspring fitness (e.g. growth rates) have been well
documented (Donohue, 2009). Maternal age at reproduction is known to
affect diapause (i.e., suspended development induced by unfavorable
environmental conditions) in offspring of insects (Mousseau & Dingle,
1991), and in amphibians, maternal factors have well known effects in
size and rates of development (Warne et al., 2013).
Maternal effects can be the result of the direct effects of the
environment on epigenetic marks, genomic imprinting, or maternal
provisioning (which is influenced by both environmental and genetic
effects). For example, the environment experienced by the mother can
affect the expression of genes involved in germination ofArabidopsis thaliana offspring (for review, see Donohue, 2009).
Genomic imprinting is the epigenetic silencing (e.g., via cytosine
methylation or chromatin-mediated processes) of one of the parental
chromosomes, leaving only expression from the non-silenced chromosome
(Alleman & Doctor, 2000). In the case of maternal effects, only the
maternal chromosomes are expressed and this can be transmitted to one or
more subsequent generations (Bischoff & Müller‐Schärer, 2010). Genomic
imprinting has been observed in a few insect species, plants and
placental mammals (for review, see Matsuura, 2020; Thamban et al.,
2020), but not in egg-laying vertebrates such as birds, monotremes and
reptiles by far (Killian et al., 2001; Renfree et al., 2013).
Maternal provisioning is the supply of nutrients, resources and hormones
by the mother during seed or egg development (Videvall et al., 2016).
For example, the amount of stored nutrient reserves in seeds can
significantly influence early seedling growth and development (Slot et
al., 2013). Maternal effects can also manifest via the seed coating
(which is maternally produced), the endosperm (which is a triploid
tissue with two-third of genotype from the maternal parent), and/or via
direct maternal effects in dispersal (Donohue, 2009). For instance,
flowering time in Campanula americana determines whether the
progeny will germinate in autumn or spring (Galloway & Etterson, 2007).
For many marine larvae, maternal provisioning of lipids is the major
source of endogenous energy and this accounts for ~40%
of the metabolic needs of coral larvae (Harii et al., 2010). Maternal
provisioning is affected by both the genotype and the environmental
conditions experienced by the mother. For example, maternal exposure to
hormones can change egg and larval morphology of reef fishes (McCormick,
1999). Maternal effects due to provisioning generally decrease over time
(Roach & Wulff, 1987), but can also persist through the entire life
cycle of an organism.
When different genotypes are combined to produce F1 (i.e., first
generation) hybrids, maternal effects can affect the phenotypes of F1
offspring. Hybridization is the crossing between separate species or
between strains/lines/populations within a species. The phenotypes of
the F1 offspring may be similar to that of their maternal parents (i.e.,
maternal effects), intermediate between the parents (i.e., additive
effects), similar to that of the dominant parent (i.e., dominance), or
different to both parents (i.e., over-dominance or under-dominance)
(Chen, 2013; Li et al., 2008; Lippman & Zamir, 2007). For example,
environmental conditions experienced by the mother can influence the
expression of genes involved of germination in progeny (Donohue, 2009).
However, hybrid gene expression studies often only involve hybrids of
one direction (Videvall et al., 2016), and hence are unable to
distinguish between dominance effects and maternal effects.
For corals, maternal effects in morphology (Willis et al., 2006),
survival (Chan et al., 2018; Isomura et al., 2013) and thermal tolerance
(Dixon et al., 2015) have been reported. Chan et al. (2018) showed that
interspecific hybrids of the corals Acropora tenuis andAcropora loripes had similar survival and growth to their
maternal purebreds, although they exceeded parental performances in some
cases. The bacterial and microalgal endosymbiont (Symbiodiniaceae spp.)
communities associated with these corals did not differ between the
reciprocal hybrids and their maternal and paternal purebreds (Chan et
al., 2019). Since these microorganisms carry vital functions to the
coral hosts and can contribute to holobiont fitness differences
(Blackall et al., 2015; Rosenberg et al., 2007), this finding suggests
that the microbial communities were unlikely responsible for the
observed holobiont fitness differences, and that these are likely
underpinned by coral host genetic and/or non-genetic transgenerational
factors.
The aim of this study was to test if the phenotypic differences in
reciprocal F1 hybrids of the corals A. tenuis and A.
loripes could be linked to patterns of host gene expression. Four
offspring groups (i.e., reciprocal F1 hybrids and two parental
purebreds) were previously produced via a laboratory cross of A.
tenuis and A. loripes and were exposed to seven months of
ambient or elevated temperature and p CO2conditions (Chan et al., 2018). Using samples from the same experiment,
we tested for maternal effects in gene expression, as observed in hybrid
survival and growth. In addition, gene expression was examined between
temperature/p CO2 conditions within each offspring
group.