Researchers have identified that Hox gene expression patterns remain largely consistent between two distinct larval developmental modes within the annelid species Streblospio benedicti. This study, published in EvoDevo in September 2024, represents the first comparative analysis of developmental divergence in Hox gene expression within a single species. By examining a marine annelid that produces offspring with different morphological and life-history traits, the team investigated how these conserved transcriptional regulators contribute to body plan variation. The findings suggest that while specific timing and location differences exist, the overall Hox gene expression framework is remarkably similar across the divergent larval types, indicating that other genetic mechanisms likely drive the major morphological differences.
The study focused on Streblospio benedicti, an estuarine annelid that exhibits intraspecific developmental dimorphism. Females of this species brood embryos in a dorsal pouch before releasing them as swimming larvae. There are two primary types of females: those producing many small eggs that develop into obligately feeding planktotrophic larvae, and those producing a few large eggs that develop into non-feeding lecithotrophic larvae. Planktotrophic larvae spend two to three weeks feeding in the plankton before becoming competent for metamorphosis, whereas lecithotrophic larvae settle to the benthos within a day of release.
Hox genes are classic examples of conserved transcriptional regulators that assign positional identity along the anterior-posterior axis in all Metazoans. They typically appear in clusters and are expressed in domains along the body axis at different developmental stages, following a pattern of spatial and temporal colinearity. While the conserved nature of Hox genes is well-known, changes in their expression can lead to evolutionary diversification and morphological novelty. The researchers aimed to determine whether differences in Hox gene expression, in either time or space, lead to the life-history and developmental changes observed in S. benedicti.
To investigate this, the team identified 11 Hox genes in the S. benedicti genome using a homologous gene approach. They assembled a transcriptome from RNA-seq data of each developmental stage in both developmental modes and used BLAST to identify homologs. The 11 Hox genes are located on chromosome 7, forming an anterior cluster and a separate posterior cluster. Molecular phylogenetic analysis confirmed that these genes are members of distinct clades containing other ortholog spiralian Hox genes, with no duplication events detected.
The researchers used hybridization chain reaction (HCR) probes for in situ hybridization to spatially and temporally visualize Hox gene expression. They also utilized RNA-seq data to quantify differential gene expression. The study examined expression from the gastrula stage through to the juvenile stage, identifying equivalent embryological and larval stages based on conserved developmental patterns. No Hox gene expression was detected at the gastrula and trochophore stages in either developmental mode.
The earliest stage where Hox gene expression was visualized was the ‘2-eye’ stage, where larvae develop two red eyes simultaneously. At this point, the first anterior body segment of the trunk appears. The researchers found that Hox gene expression patterning for both larval types is typically similar at equivalent developmental stages. However, some Hox genes exhibited spatial or temporal differences between the types. For instance, the gene Lab is expressed in the ventral area in segments 2 and 3 in late-planktotrophic larvae, swimming lecithotrophic larvae, and juveniles, but is not expressed in early-planktotrophic larvae.
Notable morphological differences arise at the ‘3-eye’ stage, where larvae add an additional anterior-lateral eye. A major difference is that only planktotrophic larvae grow ‘swimming chaetae,’ which originate from paired, lateral chaetal sacs on the first body segments. Lecithotrophic offspring never produce these swimming chaetae, although they have equivalent first chaetal sacs. The researchers observed that the genes Pb and Post1 are expressed in the swimming chaetal sacs in early-planktotrophic larvae. In contrast, in swimming lecithotrophic larvae, Pb is only found in spots in the latero-ventral body segments. This spatial difference, or heterotopy, suggests that Pb and Post1 may have been secondarily co-opted in planktotrophic larvae to regulate swimming chaetae formation.
Temporal differences, or heterochronies, were also observed. The gene Lox4 is expressed earlier in the swimming lecithotrophic larval stage than in the planktotrophic juvenile stage, although it maintains the same relative expression pattern. Post1 is expressed earlier in the 2-eye planktotrophic larval stage than in the 3-eye lecithotrophic larval stage. Additionally, Dfd and Pb stop being expressed one stage earlier in planktotrophic larvae, disappearing by the late-planktotrophic stage despite being present in swimming lecithotrophic larvae. These shifts in timing highlight the flexibility of Hox gene regulation within a single species, potentially allowing for rapid evolutionary changes in developmental timing.
The RNA-seq data showed that overall Hox gene expression is quite low but very similar between the two morphs. Some statistically significant differences in expression were indicated, primarily at the blastula stage, with later stages showing largely conserved Hox gene expression. This is consistent with other spiralian patterns of gene expression, where there is typically more variation in early embryogenesis that converges around gastrulation. The researchers noted that while RNA-seq and HCR are fundamentally different methodologies, the patterns may be similar, and bulk RNA-seq alone may not be sufficient to find spatial expression differences.
The study concludes that Hox genes are not a major contributor to the morphological and life-history differences at the level of body plan in S. benedicti. The heterochronic shifts observed in a few Hox genes are unlikely to be a major driver of body plan and life-history diversification, although they could contribute to specific morphological differences. The life-history differences and larval morphology likely arise from genes downstream of Hox expression in the developmental program.
This research provides valuable insights into the evolutionary conservation of Hox genes and their role in developmental divergence. By comparing Hox gene expression within a single species with distinct developmental pathways, the study controls for interspecific evolutionary differences, allowing for a clearer understanding of the effects of subtle changes in gene expression. The findings suggest that while Hox genes are crucial for body plan specification, their role in driving major morphological differences within a species may be limited. Further research is needed to understand the mechanisms that keep the same relative timing of expression in embryos of different sizes and ages, and how downstream genes contribute to the observed life-history differences.
These exceptions, such as the expression of Post1 and Pb in swimming chaetal sacs, indicate that Hox genes can take on different functions from the canonical expectation of specifying anterior-posterior segment identity. This flexibility in Hox gene function may be a key factor in the evolutionary diversification of body plans across different lineages.
By providing a detailed comparison of Hox gene expression within a single species, this study adds to the growing understanding of annelid Hox gene evolution. It demonstrates that while Hox genes are highly conserved, they can still exhibit variations in timing and location that contribute to specific morphological differences.
Subject of Research: Comparative Hox gene expression in dimorphic annelid larvae
Article Title: Comparative Hox genes expression within the dimorphic annelid Streblospio benedicti reveals patterning variation during development
Article References: Aguilar-Camacho, J. M., Harry, N. D., & Zakas, C. (2024). Comparative Hox genes expression within the dimorphic annelid Streblospio benedicti reveals patterning variation during development. EvoDevo, 15(1), Article 12. https://doi.org/10.1186/s13227-024-00231-5
Image Credits: AI Generated
DOI: 10.1186/s13227-024-00231-5
Keywords: Hox genes, Streblospio benedicti, annelid development, larval dimorphism, in situ hybridization, RNA-seq, developmental biology, evolutionary conservation, body plan patterning, planktotrophic larvae, lecithotrophic larvae, gene expression, morphological divergence, spatial colinearity, temporal colinearity, chaetal sacs, metamorphosis, transcriptional regulators, intraspecific variation, EvoDevo
News Source: Juliet Wilcox. (November 13, 2026). Hox Gene Expression Consistent Across Annelid Larval Types. Scienmag.
