Data from: 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.单孔目动物（Monotremes）即针鼹（echidna）与鸭嘴兽（platypus），均为卵生哺乳动物。其最独特的特征之一，在于雄性个体拥有季节性活化的毒腺（crural gland，小腿腺）。雄性鸭嘴兽会在繁殖季分泌毒液，并通过距（spur）注入其他雄性体内，用于与同类展开竞争。针鼹无法竖起其距，但该腺体会在繁殖季分泌乳白色分泌物。目前针鼹毒液的功能与分子组成仍未明确。因此，本研究将繁殖季针鼹小腿腺的深度测序转录组与鸭嘴兽的转录组进行比对，并搜寻推定毒液基因，以期为解析针鼹毒液的功能及单孔目毒液的演化历程提供线索。研究发现，针鼹毒腺转录组与鸭嘴兽存在显著差异，二者前50位高表达基因无相关性。在针鼹转录组中检测到了鸭嘴兽毒液中存在的4种肽类物质，但这些基因在针鼹体内并未呈现高表达，提示其为祖先毒腺演化历程的遗留痕迹。对针鼹前100位高表达基因进行基因本体（Gene Ontology）注释后发现，其富集的功能术语与类固醇及脂肪酸合成相关，这表明针鼹的“毒液”可能在繁殖季参与气味通讯。针鼹支系中竖起距能力的丧失，以及其他尚未明确的演化压力，导致了毒液组分的逐步退化，并使小腿腺演化出了全新的功能。