oogamy
{{Short description|Form of sexual reproduction}}
Oogamy is a form of anisogamy where the gametes differ in both size and form.
In oogamy the large female gamete (also known as ovum) is immotile, while the small male gamete (also known as spermatozoon) is mobile.{{Cite book |last1=Fusco |first1=Giuseppe |url=https://books.google.com/books?id=AKGsDwAAQBAJ|title=The Biology of Reproduction |last2=Minelli |first2=Alessandro |date=2019-10-10 |publisher=Cambridge University Press |isbn=978-1-108-49985-9 |pages=111–112 |language=en |author-link2=Alessandro Minelli (biologist)|access-date=29 March 2021}} Oogamy is a common form of anisogamy, with almost all animals and land plants being oogamous.
Oogamy is found in most sexually reproducing species, including all vertebrates, land plants, and some algae. The ancestral state of sexual reproduction is believed to be isogamy, with oogamy evolving through anisogamy. Once oogamy evolves, males and females typically differ in various aspects. Internal fertilization may have originated from oogamy, although some studies suggest {{cite journal |last1=Nozaki |first1=H |last2=Yamada |first2=TK |last3=Takahashi |first3=F |last4=Matsuzaki |first4=R |last5=Nakada |first5=T |date=2014 |title=New "missing link" genus of the colonial volvocine green algae gives insights into the evolution of oogamy |journal=BMC Evolutionary Biology |volume=14 |issue=1 |page=37 |doi=10.1186/1471-2148-14-37 |doi-access=free |pmid=24589311 |pmc=4015742 |bibcode=2014BMCEE..14...37N }}{{rp|37}} that oogamy in certain species may have evolved before the transition from external to internal fertilization. In streptophytes, oogamy occurred before the split from green algae.
Occurrence
Oogamy is found in almost all animal species that reproduce sexually.{{Cite book|last=Bell|first=Graham|url=https://books.google.com/books?id=Xzv3DwAAQBAJ&q=the+evolution+of+sexual+reproduction+is+important+to+the+evolution+of+sex|title=The Masterpiece of Nature: The Evolution and Genetics of Sexuality|date=2019-11-28|publisher=Routledge|isbn=978-1-000-49744-1|pages=63|language=en|author-link=Graham Bell (biologist)}}{{Cite book|last=Dusenbery|first=David B.|url=https://books.google.com/books?id=9hgtEAAAQBAJ&dq=oogamy+and+internal+fertilization&pg=PA326|title=Living at Micro Scale: The Unexpected Physics of Being Small|date=2009|publisher=Harvard University Press|isbn=978-0-674-06021-0|pages=32|language=en}} There are exceptions, such as the opiliones that have immobile sperm.{{Cite book|last1=Leonard|first1=Janet|url=https://books.google.com/books?id=wmlnDAAAQBAJ&dq=oogamy+in+animals&pg=PA18|title=The Evolution of Primary Sexual Characters in Animals|last2=Cordoba-Aguilar|first2=Alex|date=2010-07-19|publisher=Oxford University Press, USA|isbn=978-0-19-532555-3|pages=18–19|language=en}}
Oogamy is found in all land plants,{{Cite book|last=Simpson|first=Michael G.|url=https://books.google.com/books?id=dj8KRImgyf4C&dq=oogamy+common+in+land+animals.&pg=PA58|title=Plant Systematics|date=2010-07-19|publisher=Academic Press|isbn=978-0-08-092208-9|pages=58|language=en}} and in some red algae, brown algae and green algae.{{Cite book|last1=Raven|first1=Peter H.|url=https://books.google.com/books?id=8tz2aB1-jb4C&q=Oogamy|title=Biology of Plants|last2=Evert|first2=University Ray F.|last3=Evert|first3=Ray F.|last4=Eichhorn|first4=Susan E.|last5=Eichhorn|first5=University Susan E.|date=2005|publisher=W. H. Freeman|isbn=978-0-7167-1007-3|language=en}}{{rp|309}} Oogamy is favored in land plants because only one gamete has to travel through harsh environments outside the plant.{{rp|376}} Oogamy is also present in oomycetes.{{cite journal |first1=Anthony T. |last1=Buaya |first2=Marco |last2=Thines |journal=Philippine Journal of Systematic Biology |volume=14 |issue=1 |page=1-20 |date=2020 |title=An overview on the biology and phylogeny of the early-diverging oomycetes |doi=10.26757/pjsb2020a14004 |doi-access=free }}
Etymology
The term oogamy was first used in the year 1888.{{Cite web|title=Definition of OOGAMY|url=https://www.merriam-webster.com/dictionary/oogamy|access-date=2021-09-14|website=Merriam-Webster |language=en}} It derives from the Greek noun "oon" (ΩΟΝ = egg) and the Greek verb "gameo" (ΓΑΜΕΩ → ΓΑΜΩ = to have sex/to reproduce) and eventually means "reproduction through eggs".
Evolution
{{Main|Evolution of sexual reproduction}}
It is generally accepted that isogamy is the ancestral state{{Cite book|last1=Pitnick|first1=Scott S.|url=https://books.google.com/books?id=kctYNbO1fE0C&q=evolution+of+anisogamy|title=Sperm Biology: An Evolutionary Perspective|last2=Hosken|first2=Dave J.|last3=Birkhead|first3=Tim R.|date=2008-11-21|publisher=Academic Press|isbn=978-0-08-091987-4|pages=43–44|language=en}} and that oogamy evolves from isogamy through anisogamy.{{Cite book|last=Kumar|first=Awasthi & Ashok|url=https://books.google.com/books?id=r0h1DwAAQBAJ&dq=isogamy+generally+accepted&pg=PA363|title=Textbook of Algae|publisher=Vikas Publishing House|isbn=978-93-259-9022-7|pages=363|language=en}}{{rp|309}}{{Cite journal|last1=Hörandl|first1=Elvira|last2=Hadacek|first2=Franz|date=August 2020|title=Oxygen, life forms, and the evolution of sexes in multicellular eukaryotes|journal=Heredity|language=en|volume=125|issue=1|pages=1–14|doi=10.1038/s41437-020-0317-9|pmid=32415185|issn=1365-2540|pmc=7413252}} When oogamy has evolved, males and females typically differ in many aspects. Oogamy evolved before the transition from external to internal fertilization.{{rp|37}}{{rp|326}}
In streptophytes, oogamy occurred before the split from green algae.{{Cite journal|last1=Geng|first1=Sa|last2=De Hoff|first2=Peter|last3=Umen|first3=James G.|date=2014-07-08|title=Evolution of Sexes from an Ancestral Mating-Type Specification Pathway|journal=PLOS Biology|volume=12|issue=7|pages=e1001904|doi=10.1371/journal.pbio.1001904|issn=1544-9173|pmc=4086717|pmid=25003332 |doi-access=free }}