The study of Fibroblast Growth Factors (FGFs) in appendicularians revealed an unprecedented case of massive FGF gene losses, which played a crucial role in chordate evolution and tunicate lifestyle divergence12. This research, conducted as part of the "less, but more" evolutionary model, demonstrated how these genetic changes facilitated the transformation from sessile life forms to free-swimming organisms3. The findings provide a new framework for understanding evolutionary adaptation, suggesting that losing certain genetic elements can create opportunities for subsequent evolutionary gains and morphological changes45.

The study of Oikopleura, a genus of appendicularians, has revealed intriguing insights into cryptic speciation within these free-swimming tunicates. Cryptic species are morphologically similar but genetically distinct organisms that are often mistaken for a single species. In Oikopleura, the loss and duplication of Fibroblast Growth Factor (FGF) genes have played a crucial role in driving this hidden diversity12.

• FGF gene changes in Oikopleura have led to subtle variations in developmental patterns and physiological functions, resulting in reproductively isolated populations2.

• The "less, but more" evolutionary model helps explain how gene loss and subsequent duplications in Oikopleura have facilitated rapid adaptation to different marine environments, potentially leading to cryptic speciation34.

• This research challenges traditional views on species identification in tunicates and highlights the importance of genetic analysis in uncovering biodiversity within seemingly uniform populations of marine organisms12.

The "less, but more" evolutionary model challenges traditional views of evolution by demonstrating how gene loss can drive significant adaptations and innovations. In the case of appendicularians, the massive loss of Fibroblast Growth Factor (FGF) gene subfamilies, except for Fgf9/16/20 and Fgf11/12/13/14, led to unexpected evolutionary advancements1. This genetic simplification was followed by bursts of gene duplications within the remaining subfamilies, resulting in new functional adaptations2.

• The loss of Fgf7/10/22 and Fgf8/17/18 may be linked to the absence of drastic metamorphosis and tail absorption in appendicularians, contrasting with ascidians1.

• Gene losses opened up possibilities for function shuffling between paralogs and the innovation of new expression domains after subsequent duplications2.

• The massive co-option of Fgf genes for patterning the oikoblast, responsible for house architecture, and developing the tail fin exemplifies how gene loss can lead to novel adaptations1.

These findings highlight how evolutionary innovation can arise from genetic reduction, challenging the notion that increased complexity is always necessary for adaptive change34.

The transition from sessile to free-swimming tunicates represents a significant evolutionary leap, facilitated by the massive loss of Fibroblast Growth Factor (FGF) genes in appendicularians12. This genetic simplification paradoxically enabled complex morphological changes, allowing these organisms to adapt to a pelagic lifestyle. The study reveals how gene loss can drive major evolutionary innovations, challenging the traditional view that evolution primarily proceeds through gene acquisition and increased complexity34.

• FGF gene losses contributed to the development of a streamlined body plan suitable for active swimming

• The transition highlights the potential for evolutionary adaptation through genetic reduction

• This research provides insights into the divergence of tunicate lifestyles and the evolution of chordate body plans