The ancestors of current cytoplasmically inherited genetic material were free-living organisms (Sagan 1967), but how cytoplasmic inheritance originated and came to be limited to one sex remains an open question. For microeukaryotes (and indeed for the ancestral protoeukaryote) the presence of a microbe inside the cytoplasm would de facto produce inheritance on cell division. This would form a continuous system if replication of the microbe within the cell was occurring. Thus, the primary drive to cytoplasmic inheritance is intracellular location and replication, which could be initially host driven (symbiont capture) or symbiont driven (infection of the host, or escape from a phagolysosome).
When vertical transmission does evolve, there are two primary consequences. First, the population size of symbionts and mixing of strains declines, reducing within host conflicts. Second, symbiont fitness becomes a product not only of host survival but additionally host reproduction. Both of these processes drive the symbiosis towards the mutualism end of the mutualism-parasitism continuum, with current models indicating restrictions on symbiont diversity through bottlenecks and reduced mixing opportunities being most important in this transition, through quelling the conflicts associated with within-host competition (Leeks et al. 2019). Later transitions would then involve adaptation to the intracellular environment with correlated loss of capacity for free-living life and infection processes. All of these are reflected in the reductive genome evolution pattern commonly observed in heritable symbionts (Moran et al. 2008).
The evolution of inheritance for symbionts of multicellular hosts also has its origins in the association of free-living organisms, with a transition from symbiosis where the parties reform symbiosis through environmental association each generation to vertical transmission. Indeed, some symbiont clades include both symbionts acquired through the environment and heritable symbionts (e.g., (Drew et al. 2021)). Vertical transmission may arise passively through spatial structure (symbionts from a parent are more likely to infect progeny of that parent through proximity), actively through selection on the host to ensure passage of a beneficial symbiont (host driven vertical transmission) or actively through selection on the microbe to infect the next generation through the germ line. Evolution towards vertical transmission may be constrained through location (e.g., soil microbe-plant root associations have no proximity to the germ line), and made less likely in environments where partner availability is high (e.g., aquatic environments (Douglas 1998)) or where the symbiont has utility only in a restricted part of host life history (Hartmann et al. 2017).
There are two sides to the mutational masking that results from harbouring many copies of cytoplasmic genomes inside cells: (1) mutations with deleterious fitness effects on the host can persist longer than they otherwise would if maintained in single-copy form within the cell (Otto 2007), and (2) mutations with beneficial fitness effects on the host can be lost at higher rates because their effects are largely invisible to selection. Recent high-resolution efforts support the existence of mutational masking, as nonsynonymous mutations are more common and exist at higher frequencies than expected (Waneka et al. 2021). Moreover, masking of the fitness effects of mutations is expected to result in a deletion bias (Lawless et al. 2020), especially under relaxed selection (Wickett et al. 2008), as CIEs with replication advantages (e.g., CIEs with smaller genomes) can rise in frequency within cells rapidly (Wallace 1989; Clark et al. 2012; Sloan and Wu 2014). This latter pattern may contribute to the observation that CIEs exhibit more streamlined genomes compared to their free-living relatives (Timmis et al. 2004; Giannakis et al. 2021).
Blacker, TS; Duchen, MR; (2016) Investigating mitochondrial redox state using NADH and NADPH autofluorescence. Free Radical Biology and Medicine, 100 pp. 53-65. 10.1016/j.freeradbiomed.2016.08.010.
Bolaños, JP; Cadenas, E; Duchen, MR; Hampton, MB; Mann, GE; Murphy, MP; (2016) Introduction to Special Issue on Mitochondrial Redox Signaling in Health and Disease. Free Radical Biology and Medicine, 100 pp. 1-4. 10.1016/j.freeradbiomed.2016.08.004.
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This discussion of Western Samoa, which lies 2575 km northeast of Auckland, New Zealand, focuses on the following: geography; the people; history; government; political conditions; the economy; foreign relations; and relations the US. The population of Western Samoa, as of 1985, totals 163,000 with an annual growth rate of 0.9%. The infant mortality rate is 13/1000; life expectancy is 65 years. The main islands are formed ranges of extinct volcanoes. Volcanic activity last occurred in 1911. More than 2000 years age, waves of Polynesians migrated from Southeast Asia to the Samoan Islands. Samoans are the 2nd largest Polynesian group, after the Maoris of New Zealand, and speak a Polynesian dialect. Samoans have tended to retain their traditional ways despite exposure to European influence for more than 150 years. Most Samoans live within the traditional social system based on an extended family group, headed by a chief. Western Samoans are Christian. Education is free but not compulsory. In 1967, 95% of the children of primary school age attended school. From 1947 to 1961, a series of constitutional advances, assisted by visits from UN missions, brought Western Samoa from dependent status to self-government and finally to independence. The 1960 constitution is based on the British pattern of parliamentary democracy, modified to take Samoan customs into account. The present head of state holds his position for life. Future heads of state will be elected by the Legislative Assembly for 5-year terms. The Parliament consists of the Legislative Assembly and the head of state. The Supreme Court is the superior court of record and has full jurisdiction in civil, criminal, and constitutional matters. The \"matai\" of chief system still dominates the politics of Western Samoa, although several political parties have been formed and seem to be taking root. The \"matai\" system is a predominantly conservative force but does provide for change. Western Samoa is predominantly
Recent studies have produced new insight into the origin and distribution of some cattle ticks in the south-western Indian Ocean islands. Rhipicephalus appendiculatus, introduced from Tanzania in 2002, is now well established on Grande Comore but has not yet reached the other islands of the archipelago (Mohéli, Anjouan and Mayotte). Only one of the two clades identified in Africa has settled so far. Amblyomma variegatum, which was not supposed to be able to persist in the Antananarivo region (1300 m) nor in other Malagasy regions of high altitude without regular introductions of ticks by infested cattle, is now endemic as a general rule up to 1600 m although other regions of lower altitude (1400 m) are still free of the tick. This species remains confined in a small area of the west coast on La Reunion Island. On the contrary, Hyalomma dromedarii could not settle on Madagascar where it was introduced in 2008 and Rhipicephalus evertsi evertsi is not yet present in Grande Comore despite regular introductions by infested cattle from Tanzania. A phylogeographic approach has been carried out at an intra-specific level for A. variegatum. This study has led to the identification of two main lineages, one covering all species distribution and one restricted to East Africa and the Indian Ocean area. These two lineages are in sympatry in Madagascar where a high genetic diversity has been described, whereas a lower genetic diversity is observed on other islands. These results seem to agree with the historical data concerning the introduction of the tick in the Indian Ocean area.
A new species of free-living marine draconematid nematode, Dracograllus trukensis sp. nov., is described based on the specimens collected from the sediments of a intertidal seagrass bed from Chuuk Islands, Micronesia. Dracograllus trukensis sp. nov. differs from other species of the genus by the combination of the following characteristics: the presence of numerous minute spiny ornamented body cuticular annules in both sexes, eight cephalic adhesion tubes inserted on the head capsule in both sexes, the presence of stiff posteriorly directed setae anterior to posterior adhesion tubes in both sexes, the shape (large, elongated, open loop-shaped in male and large, elongated, closed loop-shaped in female) and position (longer ventral arm extending to the first body annule in male) of amphideal fovea, shorter spicule length (34-42 μm), the presence of sexual dimorphism in shape and length of the non-annulated tail terminus, and number of posterior sublateral adhesion tubes (10 in male and 13-15 in female) and posterior subventral adhesion tubes (8-10 in male and 9-11 in female). A comparative tabl