Live imaging of the Cryptosporidium parvum life cycle reveals direct development of male and female gametes from type I meronts

Abstract

Cryptosporidium is a leading infectious cause of diarrhea around the world associated with waterborne outbreaks, community spread, or zoonotic transmission. The parasite has significant impact on early childhood mortality, and infection is both a consequence and cause of malnutrition and stunting. There is currently no vaccine, and treatment options are very limited. Cryptosporidium is a member of the Apicomplexa, and, as typical for this, protist phylum relies on asexual and sexual reproduction. In contrast to other Apicomplexa, including the malaria parasite Plasmodium, the entire Cryptosporidium life cycle unfolds in a single host in less than 3 days. Here, we establish a model to image life cycle progression in living cells and observe, track, and compare nuclear division of asexual and sexual stage parasites. We establish the length and sequence of the cell cycles of all stages and map the developmental fate of parasites across multiple rounds of invasion and egress. We propose that the parasite executes an intrinsic program of 3 generations of asexual replication, followed by a single generation of sexual stages that is independent of environmental stimuli. We find no evidence for a morphologically distinct intermediate stage (the tetraploid type II meront) but demonstrate direct development of gametes from 8N type I meronts. The progeny of each meront is collectively committed to either asexual or sexual fate, but, importantly, meronts committed to sexual fate give rise to both males and females. We define a Cryptosporidium life cycle matching Tyzzer’s original description and inconsistent with the coccidian life cycle now shown in many textbooks.

3, 2, 1, go! Cryptosporidium counts down to sex

“…Despite its global impact, there remain major gaps in our knowledge of Cryptosporidium biology. This is well illustrated in this edition of PLOS Biology in which English and colleagues [ 4 ] discover that the current understanding of the life cycle of Cryptosporidium is likely inaccurate ( Fig 1 ). These discoveries were made possible by recent developments allowing the genetic modification of Cryptosporidium to fluorescently tag parasites, together with transcriptomic analyses to identify stage-specific genes [ 5 , 6 ] and live-cell imaging to track development.…”

“…At the third such cycle, parasites differentiate into sexually committed merozoites that reinfect a new enterocyte to form a male or female gamont . A second proliferative cycle, involving a type II meront that produces 4 merozoites, was previously believed to be required for the generation of sexually committed parasites, but English and colleagues [ 4 ] find that no such form exists in their system in C . parvum .…”

“…Using an elegant set of experiments, English and colleagues [ 4 ] start to address some of these important questions, asking first whether extrinsic or intrinsic factors drive C . parvum to undertake sexual development.…”

“…parvum are tightly synchronized. Intriguingly, a transition to sexual commitment and development occurs, almost without exception, after 3 asexual cycles [ 4 ].…”

“…parvum , despite the inclusion of this stage in the widely accepted life cycle for this parasite ( Fig 1 ). English and colleagues [ 4 ] follow up on this question through additional labeling and imaging experiments of 8N and 4N cells. In their experiments, 4N cells largely progressed to 8N type I meronts, leading to merozoites that either continued the asexual phase or developed into gamonts.…”