Ashton F T, Bhopale V M, Holt D, Smith G, Schad G A
School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104, USA.
J Parasitol. 1998 Aug;84(4):691-5.
Parasitic nematodes of the genus Strongyloides are remarkable for their ability to switch between alternative free-living developmental pathways in response to changing internal environmental conditions. After exiting the host, soil-dwelling larval stages may develop either to infectivity via 2 microbiverous stages (homogonic development) or to free-living adulthood via 4 microbiverous larval stages (heterogonic development). The progeny of these adults then give rise to the infective stage. In the latter case, free-living existence is extended in time and the number of infective larvae is greatly amplified. Anterior chemosensory neurons (amphidial neurons) are thought to respond to environmental cues and via signal transduction pathways control the direction of larval development. We now demonstrate by laser microbeam ablation that 2 classes of amphidial neurons (ASF and ASI), acting together, control the direction of free-living larval development. Larvae in which the neurons were killed developed to infectivity via the homogonic route rather than to adulthood via the otherwise predominant heterogonic route. These neurons are probable homologues of neurons ADF (=ASF) and ASI in Caenorhabditis elegans, suggesting the control of development at the cellular level is conserved among divergent taxa of nematodes. These observations also have important implications for the evolution of nematode parasitism and the design of new prophylactic measures against parasitic nematodes of medical and veterinary medical importance.
类圆线虫属的寄生线虫因其能够根据内部环境条件的变化在不同的自由生活发育途径之间切换而引人注目。离开宿主后，土壤中的幼虫阶段可以通过2个以微生物为食的阶段发育为感染性幼虫（同形发育），或者通过4个以微生物为食的幼虫阶段发育为自由生活的成虫（异形发育）。这些成虫的后代随后发育为感染性阶段。在后一种情况下，自由生活的时间得以延长，感染性幼虫的数量也大大增加。前部化学感觉神经元（双尾感器神经元）被认为对环境线索做出反应，并通过信号转导途径控制幼虫的发育方向。我们现在通过激光微束消融证明，两类双尾感器神经元（ASF和ASI）共同作用，控制自由生活幼虫的发育方向。神经元被杀死的幼虫通过同形途径发育为感染性幼虫，而不是通过原本占主导地位的异形途径发育为成虫。这些神经元可能是秀丽隐杆线虫中ADF（=ASF）和ASI神经元的同源物，这表明在不同类群的线虫中，细胞水平上的发育控制是保守的。这些观察结果对于线虫寄生的进化以及针对具有医学和兽医学重要性的寄生线虫的新预防措施的设计也具有重要意义。
