Environmental sex determination and density-dependent population regulation in the entomogenous nematodeRomanomermis culicivorax

Parasitology ◽  
1986 ◽  
Vol 92 (2) ◽  
pp. 431-449 ◽  
Author(s):  
G. A. Tingley ◽  
R. M. Anderson
Keyword(s):  
Population Growth ◽  
Sex Determination ◽  
Population Regulation ◽  
Insect Pests ◽  
Host Population ◽  
Environmental Sex Determination ◽  
Density Dependent ◽  
Regulatory Constraints ◽  
Mermithid Nematode ◽  
Low Degrees

SUMMARYEnvironmental sex determination in the mermithid nematodeRomanomermisculicivorax is examined in the context of parasite reproductive success and population regulation. Experimental results show that the sex ratio of the nematode within its mosquito host (Culex quinquefasciatus) is dependent on parasite density. Sex ratios are biased to females at low parasite burdens and to males at high parasite burdens. Low temperature further enhances female-biased ratios. The net effect of density-dependent sex determination on parasite and host population growth is shown to be critically dependent on the frequency distribution of parasite numbers/host. Mermithid parasite distributions within natural host populations show low degrees of aggregation relative to other helminth species. The population regulation of the parasite is examined with respect to environmental sex determination and parasite-induced host mortalities by means of simple mathematical models of the dynamics of parasite transmission via its life-cycle. The significance of regulatory constraints on population growth are discussed in relation to the use of mermithids as biological control agents of insect pests or disease vectors.

scholarly journals Increased natural mortality at low abundance can generate an Allee effect in a marine fish

2014 ◽  
Vol 1 (2) ◽  
pp. 140075 ◽  
Author(s):  
Anna Kuparinen ◽  
Jeffrey A. Hutchings
Keyword(s):  
Population Dynamics ◽  
Population Growth ◽  
Density Dependence ◽  
Gadus Morhua ◽  
Allee Effect ◽  
Population Regulation ◽  
Atlantic Cod ◽  
Adult Mortality ◽  
Natural Mortality ◽  
Density Dependent

Negative density-dependent regulation of population dynamics promotes population growth at low abundance and is therefore vital for recovery following depletion. Inversely, any process that reduces the compensatory density-dependence of population growth can negatively affect recovery. Here, we show that increased adult mortality at low abundance can reverse compensatory population dynamics into its opposite—a demographic Allee effect. Northwest Atlantic cod ( Gadus morhua ) stocks collapsed dramatically in the early 1990s and have since shown little sign of recovery. Many experienced dramatic increases in natural mortality, ostensibly attributable in some populations to increased predation by seals. Our findings show that increased natural mortality of a magnitude observed for overfished cod stocks has been more than sufficient to fundamentally alter the dynamics of density-dependent population regulation. The demographic Allee effect generated by these changes can slow down or even impede the recovery of depleted populations even in the absence of fishing.

scholarly journals Environmental Influences on Crustacean Sex Determination and Reproduction: Environmental Sex Determination, Parasitism, and Pollution

2020 ◽  
pp. 394-428
Author(s):  
Alison M. Dunn ◽  
Thierry Rigaud ◽  
Alex T. Ford
Keyword(s):  
Environmental Factors ◽  
Sex Ratio ◽  
Sex Determination ◽  
Environmental Influences ◽  
Host Population ◽  
Adaptive Mechanism ◽  
Environmental Sex Determination ◽  
Individual Fitness ◽  
Ratio Distortion ◽  
Sex Ratio Distortion

This chapter reviews the influences of environmental factors on sex determination, sex ratios, and reproductive behavior in the Crustacea, focusing in particular on amphipod and isopod examples. A range of abiotic and biotic environmental factors influence reproduction in Crustacea, including temperature, day length, pollutants, and parasites. Individual crustaceans may benefit from these environmental influences, but in other cases, reproductive biology responses to biotic and abiotic environments may be detrimental to individual fitness. Environmental Sex Determination (ESD) falls into the former category. ESD is an adaptive mechanism of sex determination that is rare, but has evolved in diverse taxa. Evidence from gammarid amphipods is used to explore the evolution of ESD in response to a patchy environment. While ESD is an adaptive mechanism of sex determination, the impact of other environmental factors can be very costly. Parasitic castrators can lead to a reduction or total cessation of reproduction in crustacean hosts, driving population declines. In contrast, parasitic feminizers convert male hosts into females, enhancing maternal parasite transmission but also leading to sex ratio distortion in the host population. The chapter discusses parasite-host coevolutionary conflict and reviews evidence that selection on the host in response to parasitic sex ratio distortion has led to altered mate choice in amphipods, and to the evolution of a novel system of sex determination in isopods. Human-induced environmental influences can also be seen in Crustacea, and the chapter discusses how parasites, ESD, and endocrine-disrupting chemicals can each affect sex determination and lead to abnormal intersex phenotypes. It ends by highlighting areas for future research on the diverse world of crustacean reproduction.

Density dependence and overdispersion in the transmission of helminth parasites

Parasitology ◽  
2005 ◽  
Vol 131 (1) ◽  
pp. 121-132 ◽  
Author(s):  
T. S. CHURCHER ◽  
N. M. FERGUSON ◽  
M.-G. BASÁÑEZ
Keyword(s):  
Density Dependence ◽  
Worm Burden ◽  
Anthelmintic Resistance ◽  
Parasite Burden ◽  
Host Population ◽  
Helminth Parasites ◽  
Density Dependent ◽  
Parasitic Helminths ◽  
Regulatory Constraints ◽  
Control Programmes

The influence of density-dependent processes on the transmission of parasitic helminths is determined by both the severity of the regulatory constraints and the degree of parasite overdispersion among the host population. We investigate how overdispersed parasite distributions among humans influence transmission levels in both directly- and indirectly-transmitted nematodes (Ascaris lumbricoides and Onchocerca volvulus). While past work has assumed, for simplicity, that density dependence acts on the average worm load, here we model density-dependence as acting on individual parasite burdens before averaging across hosts. A composite parameter, which we call the effective transmission contribution, is devised to measure the number of transmission stages contributed by a given worm burden after incorporating overdispersion in adult worm mating probabilities and other density-dependent mechanisms. Results indicate that the more overdispersed the parasite population, the greater the effect of density dependence upon its transmission dynamics. Strong regulation and parasite overdispersion make the relationship between mean worm burden and its effective contribution to transmission highly non-linear. Consequently, lowering the intensity of infection in a host population using chemotherapy may produce only a small decline in transmission (relative to its initial endemic level). Our analysis indicates that when parasite burden is low, intermediate levels of parasite clustering maximize transmission. Implications are discussed in relation to existing control programmes and the spread of anthelmintic resistance.

Environmental Sex Determination Mechanisms in Reptiles

2013 ◽  
Vol 7 (1-3) ◽  
pp. 95-103 ◽  
Author(s):  
H. Merchant-Larios ◽  
V. Díaz-Hernández
Keyword(s):  
Sex Determination ◽  
Environmental Sex Determination ◽  
Sex Determination Mechanisms

Population dynamics in echinococcosis and cysticercosis: regulation ofTaenia hydatigenaandT. ovisin lambs through passively transferred immunity

Parasitology ◽  
1990 ◽  
Vol 101 (1) ◽  
pp. 145-151 ◽  
Author(s):  
M. A. Gemmell ◽  
J. R. Lawson ◽  
M. G. Roberts ◽  
J. F. T. Griffin
Keyword(s):  
Population Dynamics ◽  
Natural Environment ◽  
Population Regulation ◽  
Acquired Immunity ◽  
Taenia Hydatigena ◽  
Density Dependent ◽  
Active Immunity ◽  
Infection Pressure ◽  
High Infection

SUMMARYA comparison has been made of the interactions between passively transferred and actively acquired immunity in regulating populations ofTaenia hydatigenaandT. ovis.When ewes were grazed prior to parturition under a high infection pressure, immunity was transferred to their offspring for up to 8 weeks. A qualititative difference between the species was the destruction of larvalT. ovisprior to their establishment (‘pre-encystment immunity’) and that ofT. hydatigenaafter they had become established (‘post-encystment immunity’) in the challenged lambs. The major difference in terms of population regulation between the two parasites was that infection occurred withT. hydatigenabut not withT. ovisin those lambs reared from birth for 16 weeks under high infection pressure. Passive, like active immunity, is a density-dependent constraint. It plays an important role in the population regulation ofT. ovis, but not ofT. hydatigena. This is discussed in terms of transmission in the natural environment, an hypothesis on humoral protection and the need to elucidate pathways of protection when immunization schedules are being evaluated for controlling the taeniid zoonoses.

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