Kønnet formering

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Kønnet formering kaldes det, når et nyt medlem af en art opstår ved, at arveanlæggene fra forældrene blandes. Det forstås ofte sådan, at individer inden for en bestemt art enten må være hanlige eller hunlige, men det er ikke hele sandheden. Hos planterne er det mest almindeligt, at individet bærer både hanlige og hunlige kønskarakterer. Hos svampene er individerne kønsløse, En spore kan danne en ny organisme, når hyfer fra et individ tilfældigt møder hyfer fra et andet individ, kan der dannes hyfer, der i stand til at formere sig kønnet, de fleste af de svampe vi ser har desuden den mærkværdighed at de ofte lever i lang tid med 2 cellekerner. Endelig er mange arter af snegle og orme tvekønnede, selv om de gennemfører parring.

Resultatet af en kønnet formering er, at hvert individ bliver bærer af 2 sæt kromosomer. Den kønnede formering har været en succesrig fremgangsmåde. Den er opstået hos kønsløse encellede eukaryoter, som har formeret sig med kønsløse haploide sporer. Og den har været forudsætningen for udviklingen af dyr, planter, svampe og mange andre organismer.

Det ældste bevis for kønnet formering (Bangiomorpha pubescens) findes i fossiler af flercellede organismer fra Stenianperioden i Prækambrium for ca. 1,2 milliarder år siden.^[1]^[2]^[3]

Se også

Meiose

Referencer

↑ Butterfield, Nicholas J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes". Paleobiology. 26 (3): 386-404. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2. Citat: "...Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny...Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record..."
↑ Gibson, Timothy M; Shih, Patrick M; Cumming, Vivien M; Fischer, Woodward W; Crockford, Peter W; Hodgskiss, Malcolm S.W; Wörndle, Sarah; Creaser, Robert A; Rainbird, Robert H; Skulski, Thomas M; Halverson, Galen P (2017). "Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis" (PDF). Geology. 46 (2): 135-138. doi:10.1130/G39829.1.
↑ December 20, 2017, By McGill University: Researchers Date the Origins of Photosynthesis to 1.25 Billion Years Ago, backup Citat: "...The world’s oldest algae fossils are a billion years old, according to a new analysis by earth scientists at McGill University. Based on this finding, the researchers also estimate that the basis for photosynthesis in today’s plants was set in place 1.25 billion years ago. The study, published in the journal Geology, could resolve a long-standing mystery over the age of the fossilized algae, Bangiomorpha pubescens, which were first discovered in rocks in Arctic Canada in 1990...Once the researchers had gauged the fossils’ age at 1.047 billion years, they plugged that figure into a “molecular clock,” a computer model used to calculate evolutionary events based on rates of genetic mutations. Their conclusion: the chloroplast must have been incorporated into eukaryotes roughly 1.25 billion years ago..."

Eksterne links

Ingen sex i 1,5 millioner år

[butterfield2000-1] Butterfield, Nicholas J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes". Paleobiology. 26 (3): 386-404. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2. Citat: "...Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny...Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record..."

[Cumming2017-2] Gibson, Timothy M; Shih, Patrick M; Cumming, Vivien M; Fischer, Woodward W; Crockford, Peter W; Hodgskiss, Malcolm S.W; Wörndle, Sarah; Creaser, Robert A; Rainbird, Robert H; Skulski, Thomas M; Halverson, Galen P (2017). "Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis" (PDF). Geology. 46 (2): 135-138. doi:10.1130/G39829.1.

[McGill2017-3] December 20, 2017, By McGill University: Researchers Date the Origins of Photosynthesis to 1.25 Billion Years Ago, backup Citat: "...The world’s oldest algae fossils are a billion years old, according to a new analysis by earth scientists at McGill University. Based on this finding, the researchers also estimate that the basis for photosynthesis in today’s plants was set in place 1.25 billion years ago. The study, published in the journal Geology, could resolve a long-standing mystery over the age of the fossilized algae, Bangiomorpha pubescens, which were first discovered in rocks in Arctic Canada in 1990...Once the researchers had gauged the fossils’ age at 1.047 billion years, they plugged that figure into a “molecular clock,” a computer model used to calculate evolutionary events based on rates of genetic mutations. Their conclusion: the chloroplast must have been incorporated into eukaryotes roughly 1.25 billion years ago..."

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