Background Egg 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. Results We 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). Conclusions These 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.

中文翻译：

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慈鲷中生长和应激反应相关基因的母体 mRNA 输入与卵大小和营养特化的关系。


背景 鸡蛋大小是母体效应的一种重要形式，由长期适应和平衡鸡蛋大小与雏鸡大小的短期可塑性之间复杂的相互作用决定。单染色体慈鲷是母性口育鱼，在不同物种中表现出不同的亲代投资，表现为卵大小、幼苗大小和母性护理持续时间的巨大差异。人们对鱼类卵由母系决定的分子特征及其与卵大小和营养特化的关系知之甚少。在这里，我们研究了适应坦噶尼喀湖、马拉维湖、维多利亚湖不同营养生态位的口育丽鱼鱼卵中选定的生长和应激相关基因的母体 mRNA 输入，并将它们与它们的河流盟友进行比较。结果我们首先确定了两个参考基因 atf7ip 和 mid1ip1，适合通过 qRT-PCR 对慈鲷卵中的 mRNA 丰度进行跨物种定量。使用这些参考基因，我们发现与跨物种和湖泊的生长和应激反应相关的一组候选基因的母体 mRNA 输入存在显着差异。我们观察到所有单倍色素慈鲷物种的两种生长激素受体旁系同源物（ghr1和ghr2）之间的mRNA丰度呈负相关，这也将马拉维和维多利亚湖这两个较年轻的湖泊中的物种与坦噶尼喀湖和祖先河流物种中的物种区分开来。此外，我们发现单倍色丽鱼科鱼卵大小与两个生长相关基因 igf2 和 ghr2 的母体 mRNA 丰度之间存在相关性，并且使用五个基因（ghr1、ghr2、ghr1、ghr2、 igf2、gr 和 sgk1)。 结论 这些发现表明，密切相关的慈鲷物种的卵大小变化可能与关键生长相关基因的母体 RNA 沉积差异有关。此外，具有对比营养特化的丽鱼科鱼种在其卵中沉积不同水平的母体 mRNA，以获取特定的生长相关基因。然而，目前尚不清楚这种差异是否会导致发育后期的形态发生差异。我们的结果提供了对基因激活这一方面的初步见解，作为未来针对其在生态形态专业化和适应性辐射中的作用的研究的基础。