Chapter 25: The Origin and Diversification of Eukaryotes AP Biology, Period 2 April 20, 2016

Eukaryotes arose by endosymbiosis more than 1.8 billion years ago Concept 25.1 Eukaryotes arose by endosymbiosis more than 1.8 billion years ago Pg. 481-487

Early evolution of eukaryotes: 3 major stages Pg. 482-3 Bya = billion years ago (1) Initial Diversification (2) Appearance of Novel Features (3) Rise of Large Eukaryotes

Eukaryotes contain a mixture of archaeal and bacterial genes and cellular characteristics. Pg. 484

A hypothesis for the origin of eukaryotes Endosymbiont theory: mitochondria and plasmids, including chloroplasts, originated as prokaryotic cells engulfed by host cells. The engulfed cell and its host cell then evolved into a single organism. Page 484 Endosymbiont theory

Diversity of plastids produced by endosymbiosis Other groups of protists evolved from secondary endosymbiosis events. Pg. 486 Secondary endosymbiosis

Concept 25.2 Multicellularity has originated several times in eukaryotes Pg. 487-489

Multicellular colonies The first multicellular eukaryotes were colonies, collections of cells that are connected to one another but show little or no cellular differentiation.

Independent origins of complex multicellularity Complex multicellular eukaryotes -- those with differentiated cell types -- arose independently in a variety of eukaryotic groups (including plants, fungi, animals, and several lineages of algae). Genomic analyses suggest that a transition to multicellularity from unicellular ancestors can result primarily from changes in how existing genes are used.

Concept 25.3 Four “supergroups” of eukaryotes have been proposed based on morphological and molecular data Pg. 489-498

The “supergroups” Each is a monophyletic clade Contains great diversity Most are unicellular

Excavata Excavates have unique cytoskeletal features, and some species have an “excavated” feeding groove on one side of the cell body. Include parasites as well as many predatory and photosynthetic species.

The “SAR” Clade Contains large, extremely diverse collection of protists from three major subgroups.

Archaeplastida This monophyletic group, which includes red algae, green algae, and land plants, descended from an ancient protist ancestor that engulfed a cyanobacterium.

Unikonta This clade, which is supported by studies of myosin proteins and DNA, consists of amoebozoans and opisthokonts. One current hypothesis states that unikonts may have been the first group of eukaryotes to diverge from other eukaryotes.* The opisthokonts are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa *more evidence required; root of eukaryotic tree remains unknown

Concept 25.4 Single-celled organisms play key roles in ecological communities and affect human health Pg. 499-501

Protists: key producers in aquatic communities Photosynthetic protists are among the most important producers in aquatic communities. Climate change has decreased the growth of marine producers, which could have far- reaching effects on marine ecosystems.

Symbiotic protists Protists form a wide range of mutualistic and parasitic relationships that affect their symbiotic partners and many other members of the community. Some can pose major challenges to human health. Plasmodium, the apicomplexan that causes malaria.

Questions?