Echidna venom gland transcriptome provides insights into the evolution of monotreme venom

Monotremes (echidna and platypus) are egg-laying mammals. One of their most unique characteristic is that males have venom/crural glands that are seasonally active. Male platypuses produce venom during the breeding season, delivered via spurs, to aid in competition against other males. Echidnas are not able to erect their spurs, but a milky secretion is produced by the gland during the breeding season. The function and molecular composition of echidna venom is as yet unknown. Hence, we compared the deeply sequenced transcriptome of an in-season echidna crural gland to that of a platypus and searched for putative venom genes to provide clues into the function of echidna venom and the evolutionary history of monotreme venom. We found that the echidna venom gland transcriptome was markedly different from the platypus with no correlation between the top 50 most highly expressed genes. Four peptides found in the venom of the platypus were detected in the echidna transcriptome. However, these genes were not highly expressed in echidna, suggesting that they are the remnants of the evolutionary history of the ancestral venom gland. Gene ontology terms associated with the top 100 most highly expressed genes in echidna, showed functional terms associated with steroidal and fatty acid production, suggesting that echidna “venom” may play a role in scent communication during the breeding season. The loss of the ability to erect the spur and other unknown evolutionary forces acting in the echidna lineage resulted in the gradual decay of venom components and the evolution of a new role for the crural gland.

单孔目（Monotremes）动物，即针鼹（echidna）与鸭嘴兽（platypus），均为卵生哺乳动物。该类群最具辨识度的独特特征之一，是雄性个体拥有季节性活跃的毒液/小腿腺（crural glands）。雄性鸭嘴兽会在繁殖季合成毒液，并通过距（spurs）向竞争对手传递，以辅助种内竞争。针鼹虽无法竖起其距，但在繁殖季其腺体可分泌乳状分泌物，不过针鼹毒液的功能与分子组成迄今仍未明确。为此，本研究将处于繁殖季的针鼹小腿腺的深度测序转录组（transcriptome），与鸭嘴兽的对应转录组进行比对，并搜寻潜在毒液基因，以期为解析针鼹毒液的功能及单孔目毒液的演化历史提供线索。研究发现，针鼹毒液腺转录组与鸭嘴兽存在显著差异，二者高表达排名前50的基因并无相关性。在针鼹转录组中仅检测到4种鸭嘴兽毒液中存在的肽类，但这些基因在针鼹体内的表达量并不高，提示其为祖先毒液腺演化历史的遗留痕迹。对针鼹体内高表达排名前100的基因进行基因本体（Gene Ontology）注释，结果显示其功能术语与类固醇及脂肪酸生成相关，这表明针鼹的‘毒液’或许在繁殖季的气味通讯中发挥作用。针鼹演化支系中竖起距的能力丧失，加之其他未知演化压力，最终导致毒液组分逐渐退化，并使小腿腺演化出了全新的功能。