selection
Choosey Inter-sexual females Female choice occurs, and females make good choices
Genetic benefits "Good genes" vs. "sexy son"
Fisher (1930) Runaway sexual selection Initially, males have a trait (e.g., long eye stalks) that provides a benefit. Female preference leads to a double advantage: fitness and attractiveness Positive feedback loop can lead to a maladaptive trait that ensures mating success but not necessarily fitness. "Sexy sons," but not necessarily good genes
Handicap principle (Zahavi 1975) Heritable trait with an energetic cost, reduces viability. `High-quality' males can survive despite the handicap Often expression of the trait is linked to physical condition...degree informs quality "Good genes" hypothesis this is an honest signal of fitness
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Stalk eyed flies: Diopsidae
Fit individuals can have greater handicaps They produce larger ornaments Handicap size advertises fitness Receivers' preference for large handicaps is justified
Bateman (1948) a classic study in sexual selection Bateman's principle: the sex which invests more energy in production becomes a limiting resource for which the other sex competes Therefore, there should be greater variability in reproductive success between the competing sex than the producing sex.
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Reproductive success increases for males with increasing numbers of mates, but not for females
Model works for some, but not all, mating systems. Problematic assumptions:
Sexually dimorphic patterns in variance in RS not always due to sexually passive, choosy females, and indiscriminate males.
Female RS reaches a peak after only one mating not always true. In numerous species, females that mate with multiple males have higher RS than monogamous females.
Should Females be Promiscuous?
Hypolimnas bolina
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Biological Background
The butterflies are infected with Wolbachia
Wolbachia are bacteria that infect many species of insects, other arthropods and nematodes. The infections have many effects on the host population. The effect we're interested in is altering the sex ratio by being lethal to males but harmless to females.
If there are many females and few males, what would Bateman's principle predict?
What Happens?
Greater numbers of female partners leads to fatigue in males. They start producing smaller sperm packages. Female butterflies recognize that smaller sperm packages do not fertilize all of their eggs. So, they mate more frequently.
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Modes of Insect Reproduction
Parthenogenesis
Thelytoky Arrenotoky
Sexual Reproduction
"Typical" sexual reproduction Typical sexual reproduction with polyembryony
Combined Strategies
Typical Sexual Reproduction
Offspring produced by union of two separate individuals Degree of variation between parent and offspring depends on the level of inbreeding in the population
Inbreeding common in arthropods always Not just negative effects
Polyembryony
How identical twins develop A single fertilized egg divides Some parasitoid wasps can produce hundreds of new adults from a single parasitized host
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Copidosoma (Encyrtidae) in potato moth
Combined Life Cycles
Spider mites
Aphids
Parthenogenesis (often combined with sex)
Thelytoky: unfertilized eggs develop into female adults
Arrenotoky: unfertilized eggs develop into male adults
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Tetranychus urticae
Two-spotted spider mite (Acari) Egg to adult development in 5 days at temperature >30 C Hundreds of species of host plants High density of mites leads to colonization of new plants
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Colonization Strategy
Ballooning to move Single unmated female on a plant Produces eggs that develop into males Eggs hatch and develop New adult males mate with mother Originator of colony then produces eggs that are female (fertilized) and male (unfertilized)
Alternating Strategy: Soybean Aphid
Heteroecious: alternates between more than one host plant in an annual cycle Holocyclic: has sexual reproduction in part of the annual life cycle Important pest of soybean in midwestern US Soybean summer host (Secondary host) Buckthorn winter host (Primary host)
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Distribution of Soybean Aphid
The Life Cycle
Spring
Winter host is Buckthorn (Ramnus spp.) Nymphs hatch from eggs and develop into fundatrices (singular: fundatrix) Adults produce second generation of singless, parthenogenetic females Third and later generations on Ramnus are winged parthenogenetic females Winged females move to summer host soybean
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Buckthorn
Developing Spring Colony
The Life Cycle
Summer
Many overlapping parthenogenetic generations produced Winged females (alates) are produced in response to crowding or declining nutritional value of the host plant New host plants are colonized regularly
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The Life Cycle
Autumn
Shortening photoperiod causes production of gynoparae (special winged females) that fly from soybean in search of Ramnus On Ramnus, gynoparae produce numphs that develop into oviparae (sexual females) Males are produced on soybean at the same time as gynoparae Males leave soybean in search of Ramnus and mate with oviparae Oviparae oviposit overwintering eggs on Ramnus
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Soybean Ovipara laying egg
The Life Cycle
Winter
Eggs remain in protected areas underneath bud scales until Spring cycle starts
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Why Alternate?
Costs and Benefits of Sex versus Parthenogenesis
Sex: Variation
Ability to adapt to changing conditions Exploit more different types of niches
Parthenogenesis: better numbers
Well-adapted to current conditions Exploit stable niche completely
Practical Use
SBA is imported from Asia Local Ramnus spp. are not the natural primary host Crop phenology is not exactly the same here as Asia Affects success of SBA May offer management strategies based on control by removing primary host
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