Editing Mating system (section)

===Protists===

Protists are a large group of diverse [[Eukaryote|eukaryotic]] [[microorganism]]s, mainly [[Unicellular organism|unicellular]] animals and plants, that do not form [[Tissue (biology)|tissues]]. Eukaryotes emerged in evolution more than 1.5 billion years ago.<ref name="pmid11452306">{{cite journal |vauthors=Javaux EJ, Knoll AH, Walter MR |title=Morphological and ecological complexity in early eukaryotic ecosystems |journal=Nature |volume=412 |issue=6842 |pages=66–9 |year=2001 |pmid=11452306 |doi=10.1038/35083562 |bibcode=2001Natur.412...66J |s2cid=205018792 }}</ref> The earliest eukaryotes were likely protists.  Mating and [[sexual reproduction]] are widespread among extant eukaryotes. Based on a phylogenetic analysis, Dacks and Roger<ref name="pmid10229582">{{cite journal |vauthors=Dacks J, Roger AJ |title=The first sexual lineage and the relevance of facultative sex |journal=J. Mol. Evol. |volume=48 |issue=6 |pages=779–83 |year=1999 |pmid=10229582 |doi= 10.1007/pl00013156|bibcode=1999JMolE..48..779D |s2cid=9441768 }}</ref> proposed that facultative sex was present in the common ancestor of all eukaryotes.

However, to many biologists it seemed unlikely until recently, that mating and sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that mating and sex appeared to be lacking in certain [[pathogen]]ic protists whose ancestors branched off early from the eukaryotic family tree.  However, several of these protists are now known to be capable of, or to recently have had, the capability for [[meiosis]] and hence mating. To cite one example, the common intestinal parasite ''[[Giardia lamblia|Giardia intestinalis]]'' was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex.  However, ''G. intestinalis'' was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.<ref name="pmid15668177">{{cite journal |vauthors=Ramesh MA, Malik SB, Logsdon JM |title=A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis |journal=Curr. Biol. |volume=15 |issue=2 |pages=185–91 |year=2005 |pmid=15668177 |doi=10.1016/j.cub.2005.01.003 |s2cid=17013247 |doi-access=free |bibcode=2005CBio...15..185R }}</ref> These results suggested that ''G. intestinalis'' is capable of meiosis and thus mating and sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of mating and sexual reproduction, was also found in ''G. intestinalis''.<ref name="pmid17980591">{{cite journal |vauthors=Cooper MA, Adam RD, Worobey M, Sterling CR |title=Population genetics provides evidence for recombination in Giardia |journal=Curr. Biol. |volume=17 |issue=22 |pages=1984–8 |year=2007 |pmid=17980591 |doi=10.1016/j.cub.2007.10.020 |s2cid=15991722 |doi-access=free |bibcode=2007CBio...17.1984C }}</ref> Other protists for which evidence of mating and sexual reproduction has recently been described are parasitic protozoa of the genus ''[[Leishmania]]'',<ref name="pmid19359589">{{cite journal |vauthors=Akopyants NS, Kimblin N, Secundino N, Patrick R, Peters N, Lawyer P, Dobson DE, Beverley SM, Sacks DL |title=Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector |journal=Science |volume=324 |issue=5924 |pages=265–8 |year=2009 |pmid=19359589 |pmc=2729066 |doi=10.1126/science.1169464 |bibcode=2009Sci...324..265A}}</ref> ''[[Trichomonas vaginalis]]'',<ref name="pmid18663385">{{cite journal |vauthors=Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM |title=An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis |journal=PLOS ONE |volume=3 |issue=8 |pages=e2879 |year=2008 |pmid=18663385 |pmc=2488364 |doi=10.1371/journal.pone.0002879 |bibcode=2008PLoSO...3.2879M |doi-access=free}}</ref> and [[acanthamoeba]].<ref name="pmid25800982">{{cite journal |vauthors=Khan NA, Siddiqui R |title=Is there evidence of sexual reproduction (meiosis) in Acanthamoeba? |journal=Pathog Glob Health |volume=109 |issue=4 |pages=193–5 |year=2015 |pmid=25800982 |doi=10.1179/2047773215Y.0000000009 |pmc=4530557}}</ref>

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}