<P> In normal intraspecific crossings (resulting in normal hybrids of one species), the inheritance of plastid DNA appears to be quite strictly 100% uniparental . In interspecific hybridisations, however, the inheritance of plastids appears to be more erratic . Although plastids inherit mainly maternally in interspecific hybridisations, there are many reports of hybrids of flowering plants that contain plastids of the father . Approximately 20% of angiosperms, including alfalfa (Medicago sativa), normally show biparental inheritance of plastids . </P> <P> Plastids are thought to have originated from endosymbiotic cyanobacteria . This symbiosis evolved around 1.5 billion years ago and enabled eukaryotes to carry out oxygenic photosynthesis . Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and muroplasts in the glaucophytes . The plastids differ both in their pigmentation and in their ultrastructure . For example, chloroplasts have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead contain stroma and grana thylakoids, structures found only in plants and closely related green algae . The glaucocystophycean plastid--in contrast to chloroplasts and rhodoplasts--is still surrounded by the remains of the cyanobacterial cell wall . All these primary plastids are surrounded by two membranes . </P> <P> Complex plastids start by secondary endosymbiosis (where a eukaryotic organism engulfs another eukaryotic organism that contains a primary plastid resulting in its endosymbiotic fixation), when a eukaryote engulfs a red or green alga and retains the algal plastid, which is typically surrounded by more than two membranes . In some cases these plastids may be reduced in their metabolic and / or photosynthetic capacity . Algae with complex plastids derived by secondary endosymbiosis of a red alga include the heterokonts, haptophytes, cryptomonads, and most dinoflagellates (= rhodoplasts). Those that endosymbiosed a green alga include the euglenids and chlorarachniophytes (= chloroplasts). The Apicomplexa, a phylum of obligate parasitic protozoa including the causative agents of malaria (Plasmodium spp .), toxoplasmosis (Toxoplasma gondii), and many other human or animal diseases also harbor a complex plastid (although this organelle has been lost in some apicomplexans, such as Cryptosporidium parvum, which causes cryptosporidiosis). The' apicoplast' is no longer capable of photosynthesis, but is an essential organelle, and a promising target for antiparasitic drug development . </P> <P> Some dinoflagellates and sea slugs, in particular of the genus Elysia, take up algae as food and keep the plastid of the digested alga to profit from the photosynthesis; after a while, the plastids are also digested . This process is known as kleptoplasty, from the Greek, kleptes, thief . </P>

What are the major types of plastids describe the green plastids involved in photosynthesis