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#organisms

15 public questions tagged with this topic.

Regeneration -l

Which of the following organisms shows epimorphic regeneration?

Epimorphic regeneration involves formation of regeneration blastema via dedifferentiation or stem cell recruitment, extensive proliferation under wound epidermis, and patterning akin to developmental programs. Salamanders particularly axolotl exhibit textbook epimorphic limb regeneration: amputation induces wound epidermis becoming apical epidermal cap secreting FGF8 and Wnt5a, nerve signals release Newt anterior gradient protein, underlying dermis dedifferentiates forming Prrx1-positive progenitor blastema that grows and redifferentiates into cartilage, muscle, and connective tissue. Hydra uses morphallaxis without blastema, mammals and birds form scar tissue. Salamander epimorphosis provides paradigm for nerve dependence, positional memory, and growth control.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Salamander limb epimorphic regeneration and blastema formation.

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Regeneration -l

Which of the following organisms can regenerate a lens in their eye?

Lens regeneration classic example of Wolffian transdifferentiation first described in urodeles where dorsal iris pigment epithelial cells dedifferentiate, lose melanin, re-enter cell cycle, and differentiate into transparent lens expressing crystallins. This requires retinal signals including FGF2, retinoic acid, and reactivation of Pax6. Mammals and birds possess insufficient plasticity exhibiting only capsular cataract healing, while Hydra regenerates photoreceptive structures but true lens paradigm remains amphibian. Although provided key lists Hydra as answer, textbook developmental model for functional lens regeneration remains adult newt Notophthalmus viridescens demonstrating cellular reprogramming potential relevant to regenerative medicine and stem cell induced transdifferentiation studies.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Lens regeneration via transdifferentiation - amphibian Wolffian regeneration.

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Regeneration -l

Which of the following organisms exhibits near-total regeneration ability?

Hydra, freshwater cnidarian with diploblastic body and simple nerve net, epitomizes near-total regeneration due to perpetual activity of three stem cell lineages: ectodermal epithelial, endodermal epithelial, and interstitial multipotent stem cells capable of producing neurons, nematocytes, and gametes. Small fragments containing few hundred cells reorganize polarity and reform head with hypostome organizer and basal disc within days without scarring. Zebrafish regenerate selected organs, mammals and birds show limited repair due to fibrotic responses. Hydra's continuous Wnt3-driven head organizer activity and absence of complex adaptive immunity enable whole-body reconstitution from minimal tissue.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Hydra model - interstitial stem cells and near-total regeneration.

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Transposable elements

Transposable elements are absent in

Comprehensive genome analyses reveal transposable elements populate nearly all examined organisms from bacteria to mammals, constituting large fractions of repetitive DNA. Notable exception is Plasmodium falciparum malaria parasite whose extremely AT-rich 23 Mb genome lacks active Class I or Class II transposons, perhaps due to reductive evolution, efficient elimination mechanisms, or reliance on variant gene families var and rifin diversified via recombination rather than transposition. This absence makes falciparum unique model for studying genome compaction and alternative mechanisms generating antigenic variation. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Absence of Transposable Elements in Plasmodium

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Survivorship and rK Strategies

Type III survivorship curve is typical of:

“Marine invertebrates” for type iii survivorship curve is typical of. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Selection favors the schedule that increases lifetime reproductive success under local mortality and resource conditions. Body size, development time, fecundity, parental investment, and generation length consequently tend to covary. The remaining alternatives—“Mammals”, “Birds”, “Primates”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Life-history traits reflect allocation among growth, maintenance, survival, and reproduction. Energy invested in many offspring cannot simultaneously be invested in large offspring, prolonged care, or future breeding, creating measurable trade-offs. This distinction matters because similar surface patterns can arise through different mechanisms, whereas ecological prediction depends on identifying the mechanism that actually changes rates. The cited framing is therefore most useful when treated as a conditional biological claim, with assumptions about scale and environmental context kept explicit.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 12

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Adaptation

The zone where organisms survive but with reduced reproduction is:

A tolerance curve usually has an optimum near its centre, where physiological performance, growth, and reproduction are greatest. Moving toward either the lower or upper limit enters a zone of physiological stress. Organisms can remain alive there, but energy must be diverted to homeostasis, leaving less for growth and reproduction; abundance consequently tends to be low. Beyond the tolerance limits lies the zone of intolerance, where individuals cannot survive for long and the species is absent. The “zone of extinction” is not the standard segment of a Shelford tolerance curve, while an optimal zone would show vigorous reproduction rather than reduced reproduction. Stress can arise from temperature, salinity, pH, oxygen, moisture, or any other critical factor, and the width of the curve differs among species and life stages. Reproductive stages are often more sensitive than adults, so a population may contain temporary survivors without being able to replace itself. Interacting stresses can further narrow the effective range. The zone-of-stress concept therefore distinguishes mere survival from ecological success: persistence of individual organisms does not necessarily imply a self-sustaining population under those conditions.

Ref: Evolutionary Analysis, Herron & Freeman, 5th Ed., Ch. 10

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Ecosystem Pyramid

Which organism typically has the greatest biomass in a forest ecosystem?

Trees usually contain the greatest biomass in forests because they accumulate woody tissue over decades or centuries. Their large trunks, branches, and roots store carbon far longer than the bodies of herbivores, carnivores, or parasites. Biomass is standing mass, not productivity: a forest can retain a massive producer stock even when annual net production is modest. Consumers may have rapid turnover, but respiratory losses and incomplete transfer prevent their standing biomass from approaching the long-lived plant reservoir. Energy and matter should not be conflated: nutrients can cycle among levels, but respiratory heat cannot be recycled into chemically useful energy by the community. Pyramid shape is an accounting result with biological causes, including body-size distributions, tissue longevity, consumption, assimilation, respiration, and population turnover. A snapshot may differ seasonally, especially in plankton or annual vegetation, whereas integrated production better represents ecosystem functioning across time. The ten-percent heuristic is useful for prediction but not exact; empirical transfer efficiencies vary with food quality, ectothermy, producer defenses, and detrital routing.

Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 3

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