Abstract
Cephalization refers to the evolutionary trend towards the concentration of neural tissues, sensory organs, mouth and associated structures at the front end of bilaterian animals. Comprehensive studies on gene expression related to the anterior formation in invertebrate models are currently lacking. In this study, we performed de novo transcriptional profiling on a proboscis-bearing leech (Helobdella austinensis) to identify differentially expressed genes (DEGs) in the anterior versus other parts of the body, in particular to find clues as to the development of the proboscis. Between the head and the body, 132 head-specific DEGs were identified, of which we chose 11 to investigate their developmental function during embryogenesis. Analysis of the spatial expression of these genes using in situ hybridization showed that they were characteristically expressed in the anterior region of the developing embryo, including the proboscis. Our results provide information on the genes related to head formation and insights into the function of proboscis-related genes during organogenesis with the potential roles of genes not yet characterized.
Highlights
In the development of bilaterian animals, a broadly conserved genetic toolkit underlies the specification of positional identity along the body axis, usually including the establishment of a centralized nerve system, concentration of sensory organs and development of specialized mouth parts at the most anterior region [1–3]
Among various mouthpart structures in invertebrates, the organ called the proboscis has been known as a specialized apparatus for piercing and holding the host’s gut wall [13], capturing prey through injecting toxin [14,15] and fluid sucking [16,17], and the proboscis as a feeding apparatus has a tube for sucking up food such as nectar, body fluid and cellular constituents [18,19]
We performed a comprehensive study to expand our knowledge about proboscis evolution and to discover genes related to the development of this specialized organ
Summary
In the development of bilaterian animals, a broadly conserved genetic toolkit underlies the specification of positional identity along the body axis, usually including the establishment of a centralized nerve system, concentration of sensory organs and development of specialized mouth parts at the most anterior region [1–3]. During stage 11, the proboscis gradually inverts to its resting adult position within the foregut region, by which time it consists of a complex array of longitudinal, radial and circular muscles [23,25]. These radical and dynamic developmental changes raise the question of whether essential factors are actively expressed at stage 10 for the formation of the head or specific structures such as the proboscis. Large-scale studies on expressed genes have not been reported Towards this end, we used stage 10 leech embryos and conducted RNA sequencing (RNA-seq), a successful method for understanding differential gene expression in specific tissues or under different conditions in a wide variety of animals and plants [33–35]. Our results will be used as comparative data for the evolutionary relevance of the proboscis and fill the gap in transcriptional information about the missing stage during leech embryo development
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