Abstract
BackgroundEgg size represents an important form of maternal effect determined by a complex interplay of long-term adaptation and short-term plasticity balancing egg size with brood size. Haplochromine cichlids are maternal mouthbrooders showing differential parental investment in different species, manifested in great variation in egg size, brood size and duration of maternal care. Little is known about maternally determined molecular characters of eggs in fishes and their relation to egg size and trophic specialization. Here we investigate maternal mRNA inputs of selected growth- and stress-related genes in eggs of mouthbrooding cichlid fishes adapted to different trophic niches from Lake Tanganyika, Lake Malawi, Lake Victoria and compare them to their riverine allies.ResultsWe first identified two reference genes, atf7ip and mid1ip1, to be suitable for cross-species quantification of mRNA abundance via qRT-PCR in the cichlid eggs. Using these reference genes, we found substantial variation in maternal mRNA input for a set of candidate genes related to growth and stress response across species and lakes. We observed negative correlation of mRNA abundance between two of growth hormone receptor paralogs (ghr1 and ghr2) across all haplochromine cichlid species which also differentiate the species in the two younger lakes, Malawi and Lake Victoria, from those in Lake Tanganyika and ancestral riverine species. Furthermore, we found correlations between egg size and maternal mRNA abundance of two growth-related genes igf2 and ghr2 across the haplochromine cichlids as well as distinct clustering of the species based on their trophic specialization using maternal mRNA abundance of five genes (ghr1, ghr2, igf2, gr and sgk1).ConclusionsThese findings indicate that variations in egg size in closely related cichlid species can be linked to differences in maternal RNA deposition of key growth-related genes. In addition, the cichlid species with contrasting trophic specialization deposit different levels of maternal mRNAs in their eggs for particular growth-related genes; however, it is unclear whether such differences contribute to differential morphogenesis at later stages of development. Our results provide first insights into this aspect of gene activation, as a basis for future studies targeting their role during ecomorphological specialization and adaptive radiation.
Highlights
Egg size represents an important form of maternal effect determined by a complex interplay of longterm adaptation and short-term plasticity balancing egg size with brood size
We explored the level of maternal mRNA deposition of selected components of GC and growth hormone (GH)/insulin-like growth factor pathway (IGF) pathways in the eggs of 15 haplochromine cichlid fish species with variable egg sizes and trophic specialization
Weight differences of the haplochromine cichlid eggs The egg measurements across all the species revealed a positive correlation between female length and egg weight; this was solely due to a single outlier, the Lake Tanganyika (LT) species P. famula, and the removal of this species from the analysis obliterated the correlation between these variables (Fig. 1b)
Summary
Egg size represents an important form of maternal effect determined by a complex interplay of longterm adaptation and short-term plasticity balancing egg size with brood size. Parental investment is found across the broad taxonomic range of the animal kingdom and can be biparental or exclusively uniparental It can be divided into two main categories: mating investment and rearing investment. In oviparous organisms, such as fish, maternal mating investment is significantly determined by egg size and clutch size trade-off, optimally modulated by environmental conditions [3]. This investment can affect traits functionally, developmentally, physiologically related to offspring size and survival [4] and influence later life stages [5]. The maternal phenotype and the environment she experiences hold adaptive value as they can have important implications for the fitness of offspring
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